Preheat oven to 350°F with rack in middle. Butter a 9-inch round cake pan (2 inches deep). Line bottom with a round of parchment paper, then butter parchment.

Sift together flour, baking powder, baking soda, and salt.

Beat together butter and sugar in a large bowl with an electric mixer until pale and fluffy. Beat in vanilla. Add eggs 1 at a time, beating well after each addition. With mixer at low speed, beat in buttermilk until just combined. Add flour mixture in 3 batches, mixing after each addition until just combined.

Spread batter in cake pan, smoothing top. Rap pan on counter several times to eliminate air bubbles.

Bake until golden and a wooden pick inserted in center comes out clean, 35 to 40 minutes. Cool in pan on a rack 10 minutes. Run a knife around edge of cake to loosen, then invert onto a plate. Discard paper and reinvert cake onto rack to cool completely.

Bring Sherry and sugar to a boil in a small heavy saucepan, stirring until sugar has dissolved. Put berries in a bowl and pour hot syrup over them, gently tossing to coat. Let stand 15 minutes.

Beat mascarpone and cream with sugar in a large bowl using cleaned beaters until mixture just holds stiff peaks.

Halve cake horizontally with a long serrated knife. Carefully remove top half and reserve. Put bottom half on a plate, then spread evenly with all of cream and replace top half. Serve with berries.