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1
For the lemon curd: Whisk together the egg yolks, lemon zest, lemon juice, sugar and salt in a medium saucepan.
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2
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.
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3
Remove from the heat and add the butter, one piece at a time, stirring until smooth.
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4
Transfer the curd to a heat-proof bowl and cover the surface with plastic wrap to avoid a skin from forming.
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5
Refrigerate until chilled and firm, about 30 minutes.
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6
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.
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7
In the bowl of an electric mixer, combine the cake mix, lemon zest and juice, and 1 cup water.
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8
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.
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9
With a scoop or spoon, fill the liners three-quarters full with batter.
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10
Bake for 15 minutes, or until the cupcakes are golden brown and the top feels dry.
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11
Let the cupcakes cool completely before filling and frosting, about 30 minutes.
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12
For the frosting: In the bowl of an electric mixer, combine the heavy cream, powdered sugar and vanilla.
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13
Whip until thick, about 2 minutes.
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14
Add the mascarpone and beat until stiff peaks form.
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15
To assemble: Use a small spoon to make a shallow indentation in the center of each cupcake.
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16
Fill each indentation with 1 teaspoon of the lemon curd.
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17
Use a spatula or piping bag to swirl a generous dollop of frosting over the top of each cupcake.
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18
Decorate with sprinkles or sanding sugar.
