Position racks center and preheat oven to 350°F (175°C). Grease and line three 8-inch cake pans (or two pans and bake in batches).

Whisk flour, baking powder, and salt in a bowl.

In a stand mixer, cream butter and sugar on medium-high until pale and fluffy (about 3–4 minutes). Scrape the bowl.

Add eggs one at a time, mixing well after each. Stir in vanilla.

With mixer on low, add the dry mix in three additions, alternating with the milk. Mix until just combined; don’t overmix.

Divide batter evenly into pans. Smooth tops and bake for 22–28 minutes until golden and a toothpick comes out clean. Cool in pans for 10 minutes, then invert onto racks to cool completely.

Combine raspberries and sugar over medium heat until berries break down. Add lemon juice, simmer for 3–5 minutes. Thicken with cornstarch slurry if needed. Cool completely.

Combine egg whites and sugar in a heatproof bowl over a simmering water bath. Whisk constantly until sugar dissolves and mixture reaches 160°F (71°C) or feels very warm and smooth.

Transfer to a mixer and whip on high until stiff, glossy peaks and the bowl is cool (~8–12 minutes).

Reduce speed to medium and add butter, a few cubes at a time, until smooth. Add vanilla and salt.

If it looks curdled, keep mixing—temperature will even out.

If layers domed, level with a serrated knife or cake leveler. Place first layer on a cake board, spread a thin layer of SMBC, add 1/3–1/2 cup compote (keep it thin to avoid slipping), repeat with remaining layers.

Apply a thin layer of buttercream to seal crumbs. Chill for 30 minutes until firm.

Smooth a final layer of buttercream. Pipe borders or rosettes if desired and top with fresh flowers and berries. Chill briefly to set, then bring to room temperature before serving.