Key Lime Sheet Cake Using Boxed Cake Mix

Key lime sheet cake starts with a boxed white or yellow cake mix, then gets its bright citrus flavor from lime juice and zest, with a creamy frosting on top. Baked in a rectangular pan, it is easy to slice and share; bottled Key lime juice works when fresh Key limes are hard to find.

Ingredients

1 box (about 15.25 oz / 432 g) white or yellow cake mix
1 cup (240 ml) water, or the amount specified on the cake-mix box if different
½ cup (120 ml) neutral oil, or the amount specified on the box if different
3 large eggs (about 150 g without shells), or the amount specified on the box if different
2 tablespoons (12 g) finely grated lime zest, divided
⅓ cup (80 ml) Key lime juice, fresh or bottled, divided
1 can (14 oz / 397 g) sweetened condensed milk
8 oz (227 g) cream cheese, softened
½ cup (113 g) unsalted butter, softened
3 cups (360 g) powdered sugar, sifted if lumpy
1 tablespoon (15 ml) milk or cream, plus more as needed
⅛ teaspoon fine salt
Thin lime slices or extra zest, for garnish (optional)

Instructions

Prepare the pan and oven. Heat the oven to 350°F (175°C). Grease a 9-by-13-inch (23-by-33-cm) metal baking pan, then line the bottom with parchment if you want easier removal. Follow the cake-mix directions for the listed water, oil, and eggs if its package calls for different quantities; the mix instructions take precedence over the estimates here.
Mix and bake the cake. In a large bowl, combine the cake mix, water, oil, and eggs. Beat according to the package directions, usually about 2 minutes, just until smooth. Fold in 1 tablespoon (6 g) of the lime zest. Scrape the batter into the pan and level the top. Bake for the time given on the package for a 9-by-13-inch cake, usually 25–30 minutes. It is ready when a toothpick inserted in the center comes out clean or with a few dry crumbs. Cool in the pan for 10 minutes.
Add the lime soak. While the cake cools, whisk the condensed milk with ¼ cup (60 ml) of the lime juice until smooth. Use a skewer or the round handle of a wooden spoon to make holes across the cake, spaced about 1 inch (2.5 cm) apart and reaching nearly to the bottom. Slowly spoon the lime mixture over the surface, giving it time to sink into the holes. Let the cake cool completely, about 45–60 minutes. Do not frost it while warm, or the frosting may melt.
Make the frosting. Beat the softened cream cheese and butter in a bowl until smooth and lump-free, about 2 minutes. Add the powdered sugar in two or three additions, mixing on low to avoid a cloud of sugar, then beat until fluffy. Mix in the remaining 2 tablespoons (30 ml) lime juice, the remaining 1 tablespoon (6 g) zest, salt, and 1 tablespoon (15 ml) milk or cream. If it is too stiff to spread, add more milk a teaspoon at a time; if it seems loose, beat in a little more powdered sugar.
Frost and chill. Spread the frosting evenly over the cooled cake. Garnish with lime zest or slices if desired. Refrigerate for at least 30 minutes to help the frosting firm and the lime flavor settle, then cut into 15 squares. Keep the cake chilled until serving.

Tips

Use the box as the final word on batter ratios. Cake mixes vary in size and instructions. If the package lists a different amount of water, oil, or eggs, use its quantities while keeping the lime zest addition.
Measure zest carefully. Grate only the thin green outer peel; the white pith underneath can taste bitter. Zest the limes before juicing them.
Choose your lime juice. Key lime juice has a distinctive tart, floral edge, but regular Persian lime juice works well. Bottled Key lime juice is convenient; avoid sweetened lime cordial, which can make the cake overly sweet and less tart.
Let the cake cool fully before frosting. The condensed-milk soak can make the surface look damp, but it should no longer be warm. A warm cake can loosen or melt the cream-cheese frosting.

Serving Suggestions

Serve the cake cold or let individual slices stand at room temperature for about 10 minutes so the frosting softens slightly. A small dollop of lightly sweetened whipped cream and a little extra lime zest complement the tart filling. Fresh berries, such as raspberries or strawberries, add color and a gentle fruity contrast. Because the cake is rich and tangy, modest squares are satisfying after grilled food, a picnic meal, or a casual dinner.

Variations

For a stronger citrus flavor, add another teaspoon of finely grated zest to the batter or frosting, but avoid adding much extra juice to the batter because too much liquid can affect its rise. A yellow cake mix gives a buttery flavor, while white cake mix keeps the lime taste more prominent. For a lighter topping, replace the cream-cheese frosting with whipped topping folded with a little lime zest; this is less tangy and should be kept refrigerated. To make the cake easier to transport, bake and soak it in a disposable foil pan and frost it after it has cooled completely. For a toasted garnish, sprinkle a little sweetened shredded coconut over the frosting just before serving.

Storage and Reheating

Because the frosting contains cream cheese and the cake has a milk-based soak, refrigerate it within 2 hours of serving. Cover the pan tightly or transfer slices to an airtight container; the cake keeps for 3–4 days in the refrigerator. For longer storage, freeze individual unfrosted or frosted slices, wrapped well, for up to 2 months. Thaw overnight in the refrigerator. This cake is best served chilled or slightly cool, so reheating is not recommended.

Frequently Asked Questions

Can I use regular lime juice instead of Key lime juice?
Yes. Regular fresh lime juice is a practical substitute and gives a familiar, bright lime flavor, though it is usually less floral than Key lime juice. Use the same quantity.

Can I make the cake a day ahead?
Yes. Bake, soak, cool, and frost it, then cover and refrigerate overnight. The flavors have time to settle, and the cake is easy to slice cold. Add any fresh lime slices shortly before serving.

Why did the lime soak pool on top?
The holes may have been too shallow or too widely spaced, or the mixture may have been poured too quickly. Make holes nearly to the bottom, keep them about an inch apart, and spoon the soak over gradually. A little moisture on top is normal.

Can I make this in a glass baking dish?
Yes, but glass heats differently from metal and may need a few extra minutes. Begin checking at the box’s suggested time, then continue in short increments until a toothpick comes out clean or with dry crumbs. Avoid sudden temperature changes that can crack a glass dish.

Can I use a different pan size?
A similar-volume pan can work, but baking time will change. A smaller or deeper pan takes longer and may leave the center underbaked if removed too soon; a larger pan makes a thinner cake that bakes faster. Follow the mix’s pan guidance and check for doneness rather than relying only on the clock.

Can I use low-fat cream cheese?
It can work, but the frosting may be softer and less rich. Use block-style cream cheese rather than whipped or spreadable cream cheese, which has a looser texture, and chill the frosting or finished cake if it needs to firm up.

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