Preheat oven to 350 degrees. Butter and flour two or three 8-inch or 9-inch cake pans and line with parchment paper. (See Notes)

In a small bowl, lightly whisk together the eggs, 1/2 cup of the milk, and vanilla.

In the bowl of a stand mixer fitted with a paddle attachment, combine the flour, sugar, baking powder, and salt. Add the butter, remaining 1 cup milk, and lemon zest and combine on low speed until moistened. Increase speed to medium and beat until light and fluffy, about 2 minutes, scraping down the sides of the bowl as needed.

Add the egg mixture in 3 additions, scraping down the sides of the bowl and mixing just long enough to incorporate between additions.

Divide the batter among the prepared pans and bake for 35-40 minutes or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in their pans for 10 minutes, then turn them out onto cooling racks to cool completely.

Combine sugar and lemon juice and heat over low heat on the stove, or microwave, until sugar is dissolved. Let cool.

Brush tops of cooled cake layers with simple syrup.

Wrap cake layers in plastic wrap and store in fridge until ready to assemble cake.

Add half of the blueberries, sugar, and lemon juice in a small saucepan and bring to a boil, stirring often. Reduce heat and cook for 10 minutes. Add remaining blueberries and continue to cook for 5 minutes.

Let cool completely before filling the cake.

Place a small saucepan filled with an inch of water on the stove over medium heat and bring to a boil.

Add egg whites, sugar, and salt to a heat-proof bowl and place over the boiling water. Cook for about 10 minutes, stirring occasionally, until mixture is warm and sugar is completely dissolved (rub a small amount between your fingers to test if sugar is dissolved).

Transfer egg whites to mixer bowl and beat with whisk attachment on high speed until cooled and fluffy, about 5 minutes.

Reduce mixer speed to medium high and add cubed butter a tablespoon at a time, waiting until butter is incorporated before adding more.

When all butter is added, mixture should be thick and creamy. If mixture appears curdled (butter is too cold), keep mixing until smooth, up to several minutes more. If mixture is soupy (butter is to warm), place bowl in the fridge for 20 minutes and then beat again until smooth.

Add lemon juice and mix until combined.

Place one cake layer on display stand or plate. Cover with approximately 1 cup of buttercream and half of the blueberry preserves, leaving an inch border around the preserves.

Top with next cake layer and repeat filling. Place final cake layer on top.

Frost cake with remaining buttercream and decorate as desired. See my tips for how to frost a cake.

Store cake in the fridge for at least 1 hour before serving. Assembled cake can be stored in the fridge for up to 1 week, though it will be best within 2-3 days.