I love filled cupcakes. I feel like they are this awesome surprise tucked inside this otherwise innocuous pastry. I mean, from the outside, totally a plain vanilla cupcake, jazzed up with some sprinkles. Take a bite and POW a mouthful of cool, tangy lime curd! Now my husband's work people felt the cupcakes were better without the curd. They preferred just the plain vanilla cake with the subtle lime flavor from the buttercream. Whatever floats your boat.
If you are still looking for a Valentine's treat for tonight, look no further. These cupcakes are box mix cake and with a little effort to make the curd and frosting while your cakes are baking, you have a lovely dessert wrapped up in a pretty package.
Lime Curd adapted from The Cake Bible
4 egg yolks
1/2 cup sugar
4 tablespoons butter, room temperature
1 teaspoon vanilla
3 ounces lime juice
zest from limes
Stir together egg yolks and sugar in a medium sized heavy saucepan. Pour in lime juice. Cook over medium heat until mixture thickens enough to coat the back of a spoon. Do not allow mixture to boil. It will curdle your eggs. After the mixture thickens, remove from heat. Stir in butter and vanilla. Send mixture through a fine mesh strainer and stir in lime zest. The lime curd will be yellow. This is totally fine, but if it weirds your sister out to have yellow lime curd like it does mine, then you can add a drop or two of green food coloring. Cover with plastic wrap right down on the curd to prevent a skin. Refrigerate for at least 1 hour.
Lime Buttercream adapted from Sky High Irresistible Triple Layer Cakes
4 egg whites
1 cup sugar
1/3 cup lime juice (3-4 limes)
3 sticks butter at room temperature
zest from 2 limes
In a medium sized heavy saucepan, heat up the sugar and lime juice and stir to dissolve the sugar. Bring to a boil and cook until the syrup reaches 238 degrees Fahrenheit (soft ball stage). Set up stand mixer with the egg whites and lime zest in the bowl and the whip attachment. Turn the mixer on low and beat the eggs until they are frothy. Slowly add the syrup. Continue beating on medium high speed until the egg whites have whipped up and the bowl is cool to the touch. Slowly add the butter. Continue beating, the mixture will look lumpy and broken for quite awhile. Keep beating, eventually it will become a smooth buttercream. If you add the butter before the bowl is cool, the buttercream will be very soft so be sure to wait until the bowl is cool.