Step 1: Preheat the Oven and Make the Lime Curd.

Preheat the oven to 350°F (175°C).

For the lime curd: Combine the granulated sugar, key lime juice, egg, and egg yolk in a small saucepan over medium-low heat.

Whisk until the sugar dissolves, about 1 minute.

Whisking constantly, add the chilled butter, a few pieces at a time, waiting until it melts before adding more.

Continue whisking until the curd is thick and coats the back of a spoon, 3 to 4 minutes.

Transfer the curd to a small bowl.

Refrigerate until chilled, about 1 hour.

Step 2: Prepare and Bake the Cupcakes.

Line 24 muffin cups with paper liners.

Prepare the cake mix according to the package directions for cupcakes.

Bake and cool the cupcakes as directed on the box.

Step 3: Make the Frosting.

While the cupcakes are cooling, make the frosting.

Beat the softened butter, softened cream cheese, and key lime juice in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 4 minutes.

Reduce the speed to low and slowly add the powdered sugar.

Continue to beat until just combined, 1 to 2 minutes. Don’t overmix.

Step 4: Assemble the Cupcakes.

Use a paring knife to cut a small hole in the top of each cooled cupcake, about 1/2 inch deep and 1/2 inch in diameter.

Spoon 1/2 teaspoon of the chilled key lime curd into each hole.

Spread the frosting on the cupcakes.

Sprinkle with graham cracker crumbs.

Top each cupcake with a thin lime slice.

Step 5: Serve.

Serve the cupcakes at room temperature or chilled