Position rack in middle of oven. Preheat oven to 350° F. Rub the inside of an 8-inch cake pan or springform pan with butter. Swirl flour around to coat the pan, then tap out the excess. Combine the chocolate and butter in the top of a double boiler over simmering water. Stir occasionally until the chocolate is melted. Set aside to cool. Meanwhile, sift the flour, baking powder, and salt into a bowl. Whisk together and set aside. Combine the eggs and vanilla extract in a large bowl, then whisk until foamy. Add the sugar and whisk about 1 minute or until light and frothy. Blend in the chocolate mixture. Add the flour mixture in 2 batches, whisking to blend completely each time. Pour the batter into the prepared pan. Bake 22 to 25 minutes or until a skewer inserted 1 inch from the edge comes out clean. (When the skewer is inserted in the center, a bit of moist batter should cling to it.) Cool the cake on a rack for 10 minutes, then invert onto a plate. Invert back onto rack so the shiny side is up. Cool the cake completely before sliding it onto a serving plate. Sift the cocoa over the top.