Preheat oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment; for very thin layers you can use three pans or bake in two batches. Lightly dust sides with flour and tap out excess.

Dry ingredients

In a medium bowl, whisk together the all-purpose flour, baking powder, and sea salt. Set aside.

Wet ingredients and batter

In a large bowl, whisk the granulated sugar and eggs until combined and slightly pale (about 1–2 minutes). Whisk in the vanilla. Slowly whisk in the melted butter and milk until smooth. Fold the dry ingredients into the wet mixture gently with a spatula until just combined; avoid overmixing to keep the sponge tender.

Bake thin layers

Divide batter evenly between prepared pans (for three very thin layers divide into three pans or split into two and bake one layer slightly thicker). Smooth tops. Bake 12–18 minutes, until golden and a toothpick inserted in the center comes out clean and the cake springs back when lightly touched. Rotate pans halfway through if needed for even browning.

Cool

Let layers cool in pans for 8–10 minutes, then invert onto wire racks and peel off parchment. Cool completely before assembling (about 30–40 minutes). If layers dome, level gently with a serrated knife once cool.

Berry Compote

Combine the mixed berries, granulated sugar, and lemon juice in a small saucepan over medium heat. Stir gently and bring to a simmer. Cook 6–8 minutes, stirring occasionally, until berries have softened and released juices and mixture has slightly thickened. Mash a few berries lightly with the back of a spoon for some texture. Stir in the vanilla. Remove from heat and cool to room temperature. Chill if making ahead.

Whipped Cream & Assembly

Whip the cream

Chill a mixing bowl and beaters if possible. Pour the cold heavy cream into the bowl, add powdered sugar and vanilla. Whip on medium-high speed until soft peaks form for a light, billowy cream (about 2–3 minutes). Do not overbeat — stop when the cream holds shape but is still soft.

Layer the cake

Place one cooled sponge layer on a serving plate or cake board. Spread a thin layer (about 3–4 tbs) of whipped cream, then spoon 3–4 tbs of berry compote over the cream, leaving a small border. Add the next sponge layer and repeat (whipped cream then compote). If you have three layers, repeat again. Finish with the top layer and spread the remaining whipped cream decoratively across the top and sides.

Garnish and chill

Pipe or dollop remaining whipped cream on top as desired. Arrange fresh berries on top and around the base. Dust lightly with optional powdered sugar. Chill the assembled cake for at least 30 minutes to set (up to 3 hours). Chill makes slicing cleaner.

Serve

Slice with a sharp knife dipped in hot water and wiped dry between cuts for clean slices. Serve chilled or at cool room temperature. Leftovers keep covered in the refrigerator for 2–3 days; whipped cream will soften the sponge over time.