Preheat your oven to 350°F and grease two 8-inch round cake pans with butter or baking spray. In a large bowl, whisk together the flour, baking powder, baking soda, and kosher salt to combine evenly and incorporate air into the dry mixture. This ensures your cake will have a tender crumb rather than dense pockets.
Add the room temperature butter to your dry ingredients and mix until the texture resembles coarse sand—this helps distribute the fat evenly for tender cake. Add the 4 egg whites and 1 whole egg one at a time, beating well after each addition to fully incorporate. I like to add eggs gradually rather than all at once because it helps the batter emulsify properly, which creates a better crumb structure. Finally, stir in the lemon zest for brightness.
In a small measuring cup, whisk together the vanilla extract, Greek yogurt, and milk. Pour this mixture into your batter and beat for 2 minutes until smooth and well combined. The yogurt adds moisture and tang that balances the sweetness beautifully. Divide the batter evenly between your prepared pans and bake for 25-30 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs. Cool the cakes in their pans for 15 minutes, then turn them out onto a wire rack to cool completely before frosting.
Fill a pot with about 2 inches of water and bring it to a simmer. In a heatproof bowl that sits comfortably over the pot without touching the water, combine the 5 egg whites and 2 cups sugar. Whisk the mixture constantly over the simmering water for about 8-10 minutes until the sugar completely dissolves and the mixture reaches 160°F (this pasteurizes the eggs and ensures safety). The mixture should feel smooth when you rub it between your fingers with no gritty texture.
Transfer the warmed egg white mixture to a stand mixer (or use a hand mixer) and whip for 10-12 minutes until stiff peaks form and the frosting is cool and glossy. This high-speed whipping incorporates air, creating a light, fluffy frosting. Once stiff peaks form, add the 1.5 lb of softened butter in small chunks, beating well after each addition. I like to beat in the vanilla, salt, and lemon juice at the end for a subtle brightness. The frosting may look curdled briefly—just keep beating and it will come together into a smooth, luxurious frosting. Reserve 1/2 cup of plain frosting in a small bowl, then fold the blueberry preserves and an additional 4 oz of preserves into the remaining frosting for the jam layer.
Place the first cooled cake layer on a serving plate or cake board. Using a piping bag fitted with a wide round tip, pipe a ring of blueberry jam frosting (from Step 5) around the outer edge of the cake layer. Fill the center ring with the reserved plain frosting, then top with an additional generous dollop of jam frosting in the middle. Place the second cake layer on top and cover the entire cake with a thin layer of the blueberry frosting to seal in any crumbs. Chill the cake for 30 minutes to set this crumb coat.
Remove the chilled cake from the refrigerator and apply the remaining blueberry jam frosting in a smooth, thick layer over the top and sides using an offset spatula or the back of a spoon. For a more polished look, you can create swirls or peaks as you frost. The cake is now ready to serve, or you can refrigerate it until serving time.
