Place the eggs (while still in the shell) in a bowl of warm water. This will warm them up and help them whip to a higher volume. Let them sit while you prep the cake pans and the ingredients. 

Heat oven to 350°. Grease 2 eight-inch cake pans with butter and line them with parchment paper.

Place the milk and butter in a saucepan and warm it over low heat until the butter melts. Keep the pan on the lowest heat setting to stay warm while you beat the eggs. 

Crack the eggs in a large mixing bowl and add the sugar. Beat on high speed for about 8 to 10 minutes. The eggs should triple in volume and become pale yellow in color.

Sift the flour, baking powder, and salt together. Add half of the flour to the egg mixture and fold in with a spatula until it is combined. Fold in the second half of the flour.

Add the vanilla to the warm milk mixture and gradually pour the milk into the batter. Stir the mixture at low speed. Scrape the sides and bottom of the mixing bowl with a spatula to make sure all the flour is incorporated. 

Divide the batter into the cake pans.

Bake for 25 to 32 minutes or until a toothpick comes out clean.

In a large saucepan, beat the yolks, sugar, and butter with an electric mixer until blended well.

Add the lemon juice, cornstarch (if using), and salt. Cook slowly over medium-low heat, stirring constantly until thick enough to coat the back of a spoon (if you have a candy thermometer, the mixture should be between 192° and 195°). Be patient and do not allow the mixture to boil, or it will curdle. The process will take 10 to 15 minutes.

Once thickened, pour the mixture into a strainer set over a bowl and discard any cooked egg bits that may be left in the strainer. Stir in the lemon zest, cover with plastic wrap, and refrigerate. The mixture will thicken more once chilled.

place all the ingredients in a saucepan and bring to a boil, then set aside to cool.

While the syrup boils, place the egg whites and cream of tartar in a mixing bowl. Stir to incorporate the cream of tartar and keep the mixer on low speed. Keep an eye on the candy thermometer. When the syrup reaches 230°F to 235°F, switch the mixer to medium-high speed to whip the egg whites.

When the egg whites reach soft peaks, slowly add the remaining 1/4 cup sugar and continue beating until they reach stiff peaks. As soon as the syrup reaches between 240°F to 245°F, remove it from the stove and slowly drizzle the syrup into the meringue. Keep the mixer on medium to medium-high speed while you drizzle in the syrup.

Beat on high for 2 minutes, then switch to low speed until the mixture comes to room temperature. It can take about 15 to 20 minutes for the mixture to cool down.

When all the butter is added, beat in the salt and vanilla. Continue to beat the buttercream until it is light and fluffy. 

Drizzle each of the sponge cakes with lemon syrup.

Place one of the cakes on a cake plate and spread a thin film of buttercream over the cake (this will help prevent the lemon curd from soaking into the sponge). Next, pipe a border around the perimeter of the cake to serve as a dam, so the lemon curd doesn't seep out.

Spread a little more than half of the remaining lemon curd on the first layer of the cake.

Add the second cake layer and frost the cake with the buttercream. Spread the remaining lemon curd on top of the cake.