Position oven rack in the bottom 1/3 of the oven and preheat to 375 degrees F. Spray the inside of 6 (10 ounces) ramekins, or oven-proof baking cups, with non-stick cooking spray and sprinkle with a little sugar to coat evenly.
In a large bowl, place chopped chocolate, cocoa, and 3/4 cup of the sugar.
Whisk in boiling water. When chocolate is completely melted, whisk in egg yolks and vanilla. Then stir in the flour. Set aside.
Combine egg whites with cream of tartar in a medium mixing bowl. Beat on medium speed until soft peaks form. Gradually sprinkle in remaining 1/4 cup sugar and beat on high speed until stiff but not dry.
Fold about 1/3 of the chocolate mixture into the egg whites. Fold in the remaining chocolate mixture in 2 additions. Divide batter into prepared ramekins.
Drop one 1/2 ounce of dark chocolate into the center of each souffle just before baking.
Bake for 10 to 12 minutes, or until the top has formed a nice crust and the sides appear cake-like. Sprinkle with powdered sugar. Dig in immediately!