4 ounces fine-quality bittersweet chocolate (not unsweetened)
1 stick (½ cup) unsalted butter
¾ cup sugar
3 large eggs
½ cup unsweetened cocoa powder, plus additional for sprinkling
Preheat oven to 375 degrees Fahrenheit and butter an 8-inch round baking pan. Line bottom with a round of parchment paper and butter paper.
Chop chocolate into small pieces and place into the top of a double boiler or a metal bowl set over a saucepan of barely simmering water. Add the butter and melt the chocolate and butter, stirring until smooth. Remove the top of the double boiler or bowl from heat and whisk sugar into chocolate mixture. Add eggs and whisk well. Sift ½ cup cocoa powder over chocolate mixture and whisk until just combined. Pour batter into pan and bake in middle of oven 20-25 minutes, or until top has formed a thin crust. Cool cake in pan on a rack for 5 minutes and invert onto a serving plate.
Dust cake with additional cocoa powder and serve with sorbet if desired. Cake keeps for 1 week, after being cooled completely and stored in an airtight container.