Special equipment: miniature cupcake liners, squeeze bottle, piping bag and a large star tip
Position a rack in the center of the oven and preheat to 325 degrees F. Line 3 miniature muffin tins with miniature paper liners.
Make the cake mix according to the package directions. Spoon the batter into the paper liners, filling each about halfway full (about 2 teaspoons each). Bake until a cake tester inserted into the center of a cupcake comes out clean, 12 to 15 minutes. Place the muffin tins onto a cooling rack and cool completely.
Meanwhile, whisk together the sweetened condensed milk and key lime juice. Pour the mixture into a small squeeze bottle.
Put the heavy cream and confectioners' sugar in a large mixing bowl and use a hand mixer to whip to medium-stiff peaks. Transfer the whipped cream to a piping bag fitted with a large star tip. Alternatively, if you do not have a piping bag with a star tip, you can put the whipped cream into a large resealable bag and cut a corner of the bag to create your own piping bag. Refrigerate until ready to use.
To assemble, place the tip of the squeeze bottle into the center of one cupcake. Press the tip about three-quarters of the way into the cupcake and squeeze a small amount of the key lime filling into the cupcake (the filling should just come to the top of the cupcake). Repeat this step with the remaining cupcakes.
Pipe small mounds of the whipped cream on top of each cupcake. Garnish each cupcake with a light dusting of cayenne pepper and a sprinkle of lime zest.