Combine all the ingredients in a double boiler (or in a metal bowl placed over a pot of simmering water). Heat while whisking constantly until the mixture thickens and reaches 160 degrees, or coats the back of a spoon.

Pour the lemon curd into a heatproof bowl, cover with plastic wrap pressed onto the surface of the curd to prevent a film from developing, and refrigerate until cold.

Prepare three 8-inch cake pans by lining the bottoms with parchment paper circles and greasing the sides. Preheat the oven to 350°F (176°C).

In a medium-sized bowl, combine the flour, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat together the butter, oil, sugar, and vanilla extract until light and fluffy, about 1 1/2 to 2 minutes. Do not skimp on the creaming time.

Add the eggs one at a time, mixing until mostly combined after each addition. Scrape down the sides of the bowl as needed to ensure all ingredients are fully incorporated.

Add half of the dry ingredients to the batter and mix until mostly combined.

Combine the milk and lemon juice, then gradually add this mixture to the batter and mix until fully incorporated.

Add the remaining dry ingredients and mix until well combined and smooth. Be sure to scrape down the sides of the bowl as needed. Do not overmix the batter.

Gently stir in the lemon zest to combine.

Divide the batter evenly between the prepared cake pans and bake for 22-25 minutes, or until a toothpick inserted into the center of the cakes comes out with a few crumbs.

Remove the cakes from the oven and allow them to cool in the pans for about 2-3 minutes. Then transfer them to cooling racks to cool completely.

Add the heavy whipping cream, powdered sugar, and vanilla extract to a large mixing bowl. Whip on high speed until soft peaks form.

Add the mascarpone cheese to the whipped cream and continue to whip until stiff peaks form. This process will happen fairly quickly. Set the whipped frosting in the refrigerator.

To assemble the cake, use a large serrated knife to trim the domes from the top of the cakes, if necessary.

Place the first layer of cake on a serving plate or a cardboard cake round. Pipe a dam of frosting around the outer edge of the cake using Ateco tip 808 to create a tall barrier.

Spread half of the lemon curd evenly on top of the cake layer, filling the dam about halfway full.

Add some mascarpone frosting to the top of the lemon filling and spread it into an even layer to fill the remaining space within the dam.

Add the second layer of cake and repeat the filling process with the remaining lemon curd and additional mascarpone frosting.

Place the final layer of cake on top, then smooth out the frosting around the sides of the cake.

Frost the outside of the cake, then use an offset spatula to create a striped pattern on the sides, if desired. Pipe swirls of frosting onto the top of the cake and finish by decorating with lemon slice candies and white pearl sprinkles.

Refrigerate the cake, covered, until ready to serve. The cake is best served slightly chilled, but not cold. It will stay fresh for 2-4 days when stored properly.

Tip: I recommend using mascarpone cheese that is still chilled, but soft, so it incorporates smoothly without lumps. If the mascarpone is too warm, it may soften to the point where it won't firm up properly, resulting in a too-soft frosting.