Preheat your oven to 350°F (175°C) and grease and flour three 9-inch round cake pans.

In a mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until well combined.

Cream together the unsalted butter and granulated sugar using an electric mixer until light and fluffy.

Beat in the eggs one at a time, then stir in the vanilla extract and gradually add buttermilk, alternating with the flour mixture.

Divide the batter among the three prepared pans and bake for 25-30 minutes. Let cool in pans for 10 minutes before inverting onto wire racks.

In a saucepan, combine fresh raspberries, lemon juice, and sugar over medium heat, then add cornstarch slurry and stir until thick.

Whisk egg whites and sugar in a heatproof bowl over simmering water until sugar dissolves, then beat until stiff peaks form.

Assemble by layering cake, buttercream, and raspberry filling. Repeat for second layer and top with final cake layer.

Apply a thin layer of buttercream over the entire cake and chill for at least 30 minutes to set the buttercream.

Frost the cake with a thicker layer of Swiss Meringue Buttercream and decorate as desired.