1. Preheat oven to 400 degrees F.
2. Line bottom of a buttered 8- by 2-inch round cake pan with 2 rounds of parchment paper, then butter parchment. Dust pan with some flour, knocking out excess.
3. Arrange blackberries in 1 layer in cake pan. Sprinkle berries with 1 1/2 tablespoons sugar and shake pan to help distribute sugar.
4. Whisk together 1 cup flour, baking soda, and salt in a bowl. Beat together butter and remaining 1/2 cup sugar in a large bowl with an electric mixer at high speed until light and fluffy, about 2 minutes. Add egg and vanilla and mix at low speed until just incorporated. Alternately add flour mixture and buttermilk in 3 batches, mixing at low speed until just incorporated.
5. Spoon batter evenly over berries, smoothing top, and bake in middle of oven until top is golden and a tester comes out clean, 30 to 35 minutes.
6. Run a thin knife around edge of pan, then invert a large plate over pan and, using pot holders to hold plate and pan together tightly, flip cake onto plate. Peel off parchment and serve cake with ice cream.

Yield: 6 servings