1. Set a rack in the middle of the oven and preheat to 350°. 2. Stir together the 2 1/4 cups flour, the baking powder, baking soda, and salt in a bowl. 3. In the bowl of a heavy-duty mixer, beat the butter and sugar with the paddle on medium speed until soft and light, about 5 minutes. Add the egg, beating until smooth. 4. Decrease the mixer speed to low and beat in half the flour mixture. Stop and scrape down the bowl and beater with a rubber spatula. Beat in the buttermilk, then the remaining flour mixture. 5. Give the batter a final mix with the rubber spatula. 6. Toss the currants and raisins with the remaining 1 tablespoon flour and fold them into the batter. Scrape the batter into the prepared pan and smooth the top. 7. Bake for about 45-55 minutes, or until the cake is well risen and deep gold and a toothpick inserted in the center emerges clean. 8. Cool in the pan on a rack for about 30 minutes, then invert the cake onto a rack and remove the pan and paper. Invert the cake onto another rack and cool completely. Storage: Wrap in plastic and keep at room temperature, or double-wrap and freeze for longer storage.