Break up chocolate, cut butter into chunks. In a large, heavy saucepan set over low heat, melt chocolate, butter and sugar. Let cool. Whisk egg yolks into chocolate. Whisk in flour.

Beat egg whites just until they form firm peaks. Vigorously mix one-third of the egg white into the chocolate batter. Gently and thoroughly fold in remaining whites. Pour batter into a buttered 9-inch springform pan. Bake until the cake is firm and springy, 35-40 minutes. Cool completely on a rack before releasing cake and inverting it into a platter.

Chocolate Glaze

Chop 4 oz semisweet chocolate and tumble it into a medium bowl. Heat 1/2 cup heavy cream to boiling. Pour hot cream over chocolate. Cover with a plate and let stand for 5 minutes.

With a rubber spatula, gently stir smooth. Stir in 1 tsp cognac. Slide a few strips of wax paper under the edges of the cake to catch drips. Pour all the warm glaze into a big puddle on top. Use spatula to nudge glaze toward the edges. let stand for a few minutes, then remove the wax paper.