1. Combine all the ingredients in a double boiler (or in a metal bowl over a pot of simmering water). Heat while whisking constantly until mixture thickens and reaches 160 degrees, or coats the back of a spoon.
2. Pour the lemon curd into a heat proof bowl, cover with clear wrap pressed onto the the top of the curd to avoid a film developing, and refrigerate until cold.

TO MAKE THE CAKE LAYERS:

3. Prepare three 8 inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).
2. Combine the flour, baking powder, baking soda and salt in a medium sized bowl and set aside.
3. Add the butter, oil, sugar and vanilla extract to a large mixer bowl and beat together until light in color and fluffy, about 1 1/2 to 2 minutes. Do not skimp on the creaming time.
4. Add the eggs one at a time, mixing until mostly combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Add half of the dry ingredients to the batter and mix until mostly combined.
6. Combine the milk and lemon juice, then slowly add the mixture to the batter and mix until well combined.
7. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
8. Add the lemon zest and gently stir to combine.
9. Divide the batter evenly between the cakes pans and bake for 22-25 minutes, or until a toothpick comes out with a few crumbs.
10. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.

TO MAKE THE FROSTING AND ASSEMBLE CAKE:

11. Add the heavy whipping cream, powdered sugar and vanilla extract to a large mixer bowl and whip on high speed until soft peaks form.
12. Add the mascarpone cheese to the whipped cream and whip until stiff peaks form. It will happen fairly quickly. Set whipped frosting in the refrigerator.
13. To assemble the cake, use a large serrated knife to remove the domes from the top of the cakes, if needed.
14. Place the first layer of cake on a serving plate or a cardboard cake round. Pipe a dam of frosting around the outside of the cake. I use Ateco tip 808 for the dam so that it’s tall.
15. Spread half of the lemon curd evenly on top of the cake layer, inside the dam. It should fill the dam about half way full.
16. Add some mascarpone frosting to the top of the lemon filling and spread into an even layer to fill in the remaining dam space.
17. Add the second layer of cake and repeat the filling layer with the remaining lemon curd and additional mascarpone frosting.
18. Add the final layer of cake on top, then smooth out the frosting around the sides of the cake.
19. Frost the outside of the cake, then use an offset spatula to create a striped pattern on the sides of the cake, if desired. Pipe swirls of frosting onto the top of the cake and finish it off with some lemon slice candies and white pearl sprinkles.
20. Refrigerate well covered until ready to serve. I find that the cake is best when served a little chilled, but not cold. Cake should stay fresh when well covered for 2-4 days.

*I prefer to use the mascarpone cheese when still chilled, but soft, so that it’ll in corporate well, without chunks. The warmer mascarpone cheese is, the more likely it is to soften to the point that it won’t firm up well again and can make too soft of a frosting.