Preheat oven to 375 degrees F and butter an 8-inch round baking pan. Line bottom with a round of wax paper and butter.
Chop chocolate into small pieces. In a double boiler or metal bowl set over a saucepan of barely simmering water melt chocolate with butter, stirring until smooth. Remove top of double boiler or bowl from heat and whisk sugar into chocolate mixture. Add eggs and whisk well. Sift 1/2 cup cocoa powder over chocolate mixture and whisk until just combined. Pour batter into pan and bake in middle of oven for 25 minutes, or until top has formed a thin crust. Cool cake in pan on a rack 5 minutes and invert onto a serving plate.
Dust cake with confectioners' sugar and serve heated with ice cream if desired!