I have a side bakery business, and these are one of my most popular cupcakes. If you like key lime and tart flavors, you'll love these cupcakes. It's a vanilla cupcake, a key lime curd filling, and a cream cheese frosting. People rave about these. They take a little while to make because of all the components, but it's not too bad. I usually make the cupcakes and curd one day and then make the frosting, fill, and frost the next day. These are perfect for summer, and they would be a big hit at your next BBQ.
The vanilla cupcake recipe is what I've created for high altitude, so if you're not at high altitude, don't use it. Use your favorite from scratch recipe or even a box mix. You'll make up for the box mix by making your own curd and cream cheese frosting (please don't buy that in the can--gross!). This recipe makes 12-14 but you can easily double it to make 24-28 cuppies.
Key Lime Cupcakes Recipe
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For the Cupcakes (high altitude):
1 1/2 cups cake flour
1 1/4 teaspoon baking powder
1/4 teaspoon salt
8 tablespoons butter
1 cup minus 3 tablespoons sugar
2 eggs plus 1 egg white
1 cup buttermilk
1/4 cup instant vanilla pudding mix
For the Key Lime Curd (any altitude):
1/4 cup key lime juice (about 10 key limes)
1/2 tablespoon key lime zest
2 tablespoons + 2 teaspoons sugar
1/4 cup butter, softened and cut into two pieces
For the Cream Cheese Frosting (any altitude):
3/4 package cream cheese (6 ounces)
6 tablespoons butter
12 ounces powdered sugar (about 2 3/4 cups)
1/8 teaspoon salt
1 teaspoon vanilla
1 lime with rind removed, sliced, and twisted
To make the high-altitude cupcakes, preheat your oven to 375 degrees and place cupcake liners in your cupcake pan(s). Mix together the flour, baking powder, salt, and instant vanilla pudding mix and set aside. In a mixing bowl fitted with the paddle attachment, cream together your butter and sugar. Add the eggs one at a time until incorporated. Scrape down the sides of the bowl. With the mixer on low speed, add the flour mixture and buttermilk in two additions, starting and ending with the dry ingredients. Mix on low until incorporated, scrape down the sides of the bowl, and mix for another 10-15 seconds. Divide the batter into the prepared muffin liners. You want to fill each liner about 2/3 full.
Bake for 15-17 minutes or until a toothpick inserted into the middle comes out clean. Cool in the pans for 5-10 minutes and remove to a wire rack. Cool completely before filling and frosting.
If you're not a high altitude, use your favorite scratch vanilla cupcake recipe or box mix and cool completely before proceeding.
To make the key lime curd, whisk together the key lime juice, zest, egg, and sugar in a mixing bowl set on top of a saucepan with simmering water (this is the double boiler method). Add butter and whisk until thickened and smooth (a thermometer would read 160-170 degrees). Strain through a fine mesh strainer into a clean bowl and cover with plastic wrap to chill. It will pipe into your cupcakes much easier if it's chilled.
To make the cream cheese frosting, in your mixing bowl fitted with the paddle, cream together the butter and cream cheese until smooth. Add the vanilla and salt and mix until incorporated. Add the powdered sugar one cup at a time and mix until smooth. It's best to weigh your powdered sugar because your amount in cups will depend on your humidity. If you don't have a kitchen scale, you'll want the frosting to be thick enough to pipe onto the cupcakes. Store leftover frosting in the fridge or freezer.
To assemble the cupcakes, use an apple corer or a knife to cut a hole in the center of your cupcakes. You don't need to go to the very bottom of the cupcake. Fill the hole with the chilled key lime filling. You can do this with a spoon, a pastry bag, or a sandwich bag with the corner snipped off. You don't need to place cake over the hole as the frosting will cover it. Pipe frosting on top of the cupcake. Garnish with lime twists or sprinkles.
Cupcakes adapted from Annie's Eats
Key Lime Curd adapted from Confections of a Foodie Bride
Cream Cheese Frosting from The Pioneer Woman Cookbook