Preheat oven to 350°F (175°C) and prepare three 8-inch round cake pans with parchment and non-stick spray.

Combine fresh lemon juice and whole milk in a medium bowl and let it sit for 15 minutes.

In a large bowl, mix granulated sugar with lemon zest, then sift in cake flour, baking powder, and salt. Whisk together.

In a separate bowl, whisk together curdled milk, eggs, egg whites, sour cream, vegetable oil, and vanilla.

Combine wet and dry mixtures, mixing gently until just combined.

Divide batter among prepared pans and bake for 28-32 minutes, until a toothpick comes out clean.

Cool cake layers in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Make buttercream by beating softened butter until creamy, then gradually add powdered sugar and mix in lemon extract and milk as needed.

Frost cooled cake layers, decorating with dried lemon slices if desired.