1. Preheat oven to 350 degrees, setting a rack in the middle level. Butter a 10-inch round cake pan, 2 inches deep, and cut a piece of parchment or wax paper to fit the bottom.

2. Chop the chocolate finely and place it in a small bowl over hot water to melt, stirring occasionally. Remove the bowl from the pan and cool the chocolate slightly.

3. In a large mixer bowl, beat the butter with half the sugar until it’s soft and light. Beat in the chocolate, then the yolks, one at a time, scraping the bowl and beater(s) often. Continue beating until the mixture is smooth and light.

4. Place the almonds in the bowl of a food processor and grind them finely, pulsing the machine on and off at 1-second intervals. Be careful that they do not become pasty. Stir the almonds and amaretti crumbs, then the flour, into the batter.

5. In a clean, dry bowl, whip the egg whites until they hold a very soft peak and whip in the remaining sugar in a slow stream. Continue to whip the whites until they hold a soft, glossy peak. Stir a quarter of the whites into the batter, then fold in the rest with a rubber spatula, so that no streaks remain.

6. Pour the batter into the prepared pan and smooth the top with a spatula. Bake the cake about 40 minutes, until the center is firm when pressed with a fingertip.

7. Cool the cake in the pan for 5 minutes. The cake may sink slightly, though this does not affect its texture. Trim off any loose crust and invert the cake on a rack, remove the pan, and allow to cool completely.

8. Dust the cake with confectioners’ sugar and slide onto a platter.