1. Heat the oven to 350°F. Butter a 9-inch layer cake pan. Cover the bottom with a circle of wax or parchment paper and butter the paper. Use a sieve to dust the pan with flour or cocoa powder, invert the pan over the sink, and tap to remove any excess.
2. Whisk the flour and salt together in a medium bowl. Melt the chocolate with 1/2 cup water in the top of a double boiler, stirring occasionally. Remove from the heat and let cool.
3. In a large bowl, use an electric mixer to beat the eggs and vanilla until light. Gradually add the sugar, continuing to beat until the mixture is very thick, 8 to 12 minutes. Gently sift half the flour mixture over the egg mixture and stir it in, followed by the melted chocolate. Then sift and mix in the remaining flour, being careful not to overmix and deflate the eggs.
4. Pour the batter into the prepared pan. Bake until a toothpick inserted into the center of the cake comes out clean or with a few moist crumbs, 30 to 40 minutes. Let the cake cool in the pan for 5 minutes, then invert onto a wire rack to finish cooling. Dust the cake with confectioners’ sugar if you like and serve small slices.

– Recipe from How to Cook Everything: Completely Revised Twentieth Anniversary Edition