The mascarpone in the frosting makes for a creamy delight and complements the sweet lemon. Try this cupcake for something different than your traditional chocolate or vanilla flavor.
Whisk together the yolks, lemon zest, juice, sugar and salt in a medium saucepan. Place the saucepan over medium-low heat, stirring constantly with a wooden spoon or spatula, making sure to scrape the sides and bottom of the pan. Cook until the mixture is thick enough to coat the back of the spoon or spatula, about 3 to 5 minutes. Remove from the heat and add the butter, one piece at a time, stirring until the consistency of the curd is smooth. Transfer the curd to a heat-proof bowl and cover the surface of the curd with plastic wrap to avoid a skin from forming. Refrigerate the curd until firm and chilled, about 1 hour.
Place two racks in the upper and lower thirds of the oven and preheat the oven to 350 degrees. Line 2 muffin pans with cupcake liners. In the bowl of an electric mixer combine the cake mix, water, lemon zest and juice. Beat the mixture on low speed to for 30 seconds to incorporate the ingredients then increase the speed to medium for 1 minute until the mixture is light and fluffy. Use a scoop or spoon to fill the liners 3/4 full with the batter. Bake for 12 to 15 minutes, until the cupcakes are golden brown and the cracks on top feel dry. Cool the cupcakes completely before filling and frosting, about 30 minutes.Use a small spoon to push a shallow hole in the center of each cupcake.
Whisk together the powdered sugar, mascarpone, milk, and vanilla until smooth and shiny. Fill the center with 1 tsp of the lemon curd. Drizzle the cupcake with some of the glaze to cover the top. Sprinkle with the decoration of your choice. Allow the cupcakes to set, in the refrigerator, for at least 30 minutes.