Preheat oven to 400°F with rack in middle. Butter and flour a 9-inch round cake pan.
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Whisk together flour, baking powder, baking soda, and salt and set aside. In a larger bowl, beat butter and 2/3 cup (146 grams) sugar with an electric mixer at medium-high speed until pale and fluffy.
Step 3 of 6
Add egg and beat well. Also beat in vanilla and zest, if using.
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At low speed, mix in flour mixture in three batches, alternating with buttermilk, beginning and ending with flour, and mixing until just combined.
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Spoon batter into cake pan, smoothing top. Scatter (see Note) raspberries evenly over top and sprinkle with remaining 1 1/2 tablespoons (22 grams) sugar.
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Bake until cake is golden and a wooden pick inserted into center comes out clean, 20 to 25 minutes. Cool in pan 10 minutes, then turn out onto a rack and cool to warm, 10 to 15 minutes more. Invert onto a plate.