Position the rack in the middle of the oven. Preheat the oven to 350°.
Rub the inside of an 8-inch cake pan or a springform pan with butter. Swirl a few teaspoons of flour around to coat completely. Invert the pan and tap out the excess.
Sift the flour, baking powder, and salt into a bowl; whisk well and set aside.
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, combine the eggs, vanilla extract in a large bowl. Whisk until foamy. Add the sugar and whisk until light and frothy, 1 minute. 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 until a skewer inserted 1 inch from the edge comes out clean. When inserted in the center, a bit of moist batter will cling to it. Do not over bake.
Cool the cake on a rack 10 minutes, then invert onto a plate. Invert back onto the rack so the shiny side is up. Cool the cake completely before sliding it onto a serving plate. Sift the cocoa over the top.
