This is excellent and easy:
FLOURLESS CHOCOLATE CAKE
4 ounces bittersweet (baking) chocolate
1 stick butter (½ cup)
¾ cup sugar
3 eggs (room temperature)
½ cup unsweetened cocoa powder
1. Preheat oven to 375 degrees.
2. Butter 8-inch round cake pan; line bottom with an 8-inch
round of wax paper and butter the paper.
3. Break chocolate into pieces and put in glass measuring
cup; microwave on high about 30 seconds, remove from
microwave and stir; cut butter into small pieces and add to
measuring cup; microwave another 30 seconds on high,
remove from microwave and stir; if chocolate has not
melted completely, return to microwave for 10 seconds,
then stir again; repeat until chocolate is just barely melted;
set aside to cool (do not use while still hot).
4. Put chocolate mixture in large mixing bowl; add sugar and
whisk until very smooth.
5. Add eggs and whisk well, about 1 to 2 minutes.
6. Sift cocoa over mixture in bowl; fold together with a rubber
spatula until just combined.
7. Pour batter into prepared pan and bake 25 minutes, or
until a thin crust forms on top; cake is done when it just
begins to pull away from sides of pan.
8. Cool in pan 5 minutes, then invert on serving plate.
9. Dust with more cocoa or powdered sugar before cutting
into serving pieces. Serve with vanilla ice cream or
whipped cream.