Make the Lemon Curd

1. Whisk the eggs, sugar, cornstarch (if using), lemon juice, and lemon zest in a heatproof bowl until smooth.
2. Cook over low to medium heat in a double boiler or directly in a saucepan, whisking constantly.
3. Continue cooking for 15 to 20 minutes until the lemon curd thickens and coats the back of a spoon.
4. Remove from the heat and gradually whisk in the softened butter until smooth and glossy.
5. Strain through a fine-mesh sieve to remove the zest and any cooked egg bits.
6. Transfer to a clean bowl or jar and cover with plastic wrap directly touching the surface.
7. Chill completely in the refrigerator before using.

Make the Lemon Mascarpone Frosting

1. Place the cold mascarpone, cold heavy cream, cold lemon curd, powdered sugar, and vanilla extract in a large mixing bowl.
2. Whip on low to medium speed for about 2 minutes until thick, smooth, and firm.
3. Use immediately to fill or frost cakes, cupcakes, and desserts, or refrigerate until ready to use.