Ingredients

½ cup granulated sugar

¼ cup bottled Key lime juice

1 large egg

1 egg yolk

2 Tbsp salted butter, cut into pieces and chilled

1 (15–16 oz.) box white cake mix (plus required ingredients per box, usually eggs, oil, and water)

1 stick (½ cup) salted butter, softened to room temperature

8 oz. cream cheese, softened to room temperature

¼ cup bottled Key lime juice

4 cups powdered sugar (approximate; adjust to taste/consistency)

Graham cracker crumbs

Thin lime slices (optional)

Instructions

Step 1: Prepare the Key Lime Curd by whisking together sugar, Key lime juice, 1 whole egg, and 1 egg yolk in a small saucepan. Place over medium-low heat and stir constantly for 5–7 minutes, until thick enough to coat the back of a spoon. Remove from heat and stir in chilled butter pieces until melted. Transfer to a bowl, cover with plastic wrap pressed directly on the surface, and refrigerate until cool.

Step 2: Make the Cupcakes by preheating the oven to the temperature indicated on your cake mix box (usually 350°F/177°C). Line two standard cupcake pans with paper liners. Prepare the white cake batter according to package instructions. Fill each liner about two-thirds full and bake for 18–22 minutes or until a toothpick comes out clean. Let cool in the pan briefly, then transfer to a wire rack to cool completely.

Step 3: Prepare the Cream Cheese Frosting by beating the softened butter and cream cheese together on medium speed until smooth, about 2 minutes. Add Key lime juice, then gradually beat in powdered sugar on low speed. Increase to medium-high and beat until light and fluffy, adjusting sweetness or lime juice to taste.

Step 4: Assemble and Decorate by using a small knife or apple corer to remove a small well from the center of each cooled cupcake. Spoon or pipe about a teaspoon of chilled Key lime curd into each well. Pipe or spread the cream cheese frosting on top. Sprinkle with graham cracker crumbs and garnish with thin lime slices if desired.

Tips & Variations

For a more intense lime flavor, use fresh Key lime juice if available instead of bottled.

Chilling the curd completely before filling ensures it stays in place and adds a nice texture contrast.

Try using crushed graham crackers mixed with a bit of melted butter as a cupcake topping for extra richness.

If you prefer less sweet frosting, reduce the powdered sugar gradually while tasting.

Storage

Store these cupcakes in an airtight container in the refrigerator for up to 3 days to keep the cream cheese frosting and curd fresh. Let them come to room temperature for about 20 minutes before serving for the best flavor and texture. They can also be frozen without frosting for up to 1 month; thaw completely before frosting.

FAQs

Can I make the Key Lime Curd ahead of time?

Yes, the curd can be made up to 3 days in advance and kept refrigerated. Just cover it well to prevent it from drying out or absorbing other fridge odors.

What can I use if I don’t have Key limes?

You can substitute regular lime juice or even lemon juice, but the flavor will be slightly different. Key limes have a unique sweet-tart taste that’s hard to replicate exactly.