Preheat oven to 350F. Butter two 9-inch round cake pans, line bottoms with parchment, and butter the parchment. Whisk flour, baking powder, and salt together in a bowl.

Beat butter and sugar on medium-high speed until pale and fluffy, about 4 minutes. Scrape down bowl. Add eggs one at a time, beating well after each, then beat in vanilla.

With mixer on low, add flour mixture in three additions alternating with milk in two additions, beginning and ending with flour. Mix just until smooth - do not overbeat.

Divide batter between pans and smooth tops. Bake 28-32 minutes until cakes spring back when touched and a toothpick comes out clean. Cool in pans 15 minutes, then turn out onto racks to cool completely.

Toss berries with 2 tablespoons sugar in a bowl. Let sit at room temperature 30 minutes until juicy and slightly softened. Drain excess liquid before assembling (save it for drizzling if desired).

Beat heavy cream on medium-high until soft peaks form. Add mascarpone, powdered sugar, and vanilla. Beat on medium until stiff peaks form and cream holds its shape - about 2 minutes. Do not overbeat or it will become grainy.

Place one cake layer on a platter. Spread with 1 cup chantilly cream and top with half the berries. Add second cake layer. Frost top and sides with remaining cream. Pile remaining berries on top, letting some tumble down the sides. Chill at least 30 minutes before slicing.