For the lemon curd: Whisk together the egg yolks, lemon zest, lemon juice, sugar and salt in a medium saucepan. Set the saucepan over medium-low heat and cook, stirring constantly with a wooden spoon or spatula and making sure to scrape the sides and bottom of the pan, until the mixture is thick enough to coat the back of a spoon, 3 to 5 minutes. Remove from the heat and add the butter, one piece at a time, stirring until smooth. Transfer the curd to a heat-proof bowl and cover the surface with plastic wrap to avoid a skin from forming. Refrigerate until chilled and firm, about 30 minutes.
For the cupcakes: Set racks on the upper and lower thirds of the oven and preheat to 350 degrees F. Line two muffin pans with cupcake liners. In the bowl of an electric mixer, combine the cake mix, lemon zest and juice, and 1 cup water. Beat on low speed for 30 seconds to incorporate, then raise the speed to medium and beat until the mixture is light and fluffy, about 1 minute. With a scoop or spoon, fill the liners three-quarters full with batter. Bake for 15 minutes, or until the cupcakes are golden brown and the top feels dry. Let the cupcakes cool completely before filling and frosting, about 30 minutes.
For the frosting: In the bowl of an electric mixer, combine the heavy cream, powdered sugar and vanilla. Whip until thick, about 2 minutes. Add the mascarpone and beat until stiff peaks form.
To assemble: Use a small spoon to make a shallow indentation in the center of each cupcake. Fill each indentation with 1 teaspoon of the lemon curd. Use a spatula or piping bag to swirl a generous dollop of frosting over the top of each cupcake. Decorate with sprinkles or sanding sugar.