For the lemon cake: Preheat the oven to 325 degrees F. Line two 9-inch round cake pans with parchment paper and spray with nonstick cooking spray.
Whisk together the flour, baking powder and salt.
With an electric mixer on medium-high speed, cream together the butter and sugar until pale and fluffy. Add the eggs, one at a time, beating until each is incorporated and scraping down the sides of the bowl as needed. Beat in the lemon zest and vanilla.
Add the flour mixture in 3 batches, alternating with two additions of milk and lemon juice and beating until just combined after each. Divide the batter evenly between the prepared cake pans.
Bake, rotating halfway through, until golden brown and a cake tester inserted in the center comes out clean, 25 to 30 minutes. Cool in the pans on a rack for 5 to 10 minutes, then remove the cake layers from the pans and wrap in plastic wrap to cool completely.
For the blueberry compote: Meanwhile, bring 1 cup of the blueberries and the sugar, lemon juice and water to a simmer in a small saucepan over medium heat. Cook until the berries burst and the liquid thickens slightly, about 5 minutes. Stir in the remaining 1/2 cup blueberries and let cool.
For the buttercream: Wipe the bowl of a stand mixer with a paper towel dampened with lemon juice, to remove any trace of grease.
Add the egg whites and sugar to the bowl, and cook over a pot of simmering water, whisking constantly but gently, until hot and the sugar has dissolved. Transfer the bowl to the stand mixer fitted with the whisk attachment and begin to whip until the meringue is thick and glossy, and the bottom of the bowl feels room-temperature to the touch, about 10 minutes. Switch to the paddle attachment and, with the mixer on low speed, add the butter cubes 1 at a time until incorporated; then mix until silky smooth. Add the vanilla and salt, continuing to beat on low speed until well combined. Slowly add up to 1/2 cup blueberry compote, mixing until the desired texture is reached.
To assemble: Top a cake layer with blueberry compote, then top with the remaining cake layer. Frost all over with the buttercream. Serve immediately, or refrigerate and bring the cake back to room temperature for 15 minutes before serving.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.