Step 1Grate the chocolate and place it in a bowl. Bring the whipping cream just to a boil and pour it over the chocolate. Slowly stir until smooth. Transfer the mixture into a shallow glass dish and let it cool slightly, about 10 minutes, the place it in the freezer to chill, 1 hour.
Step 2Scoop out 10 balls using a 1/2-teaspoon measuring spoon and shape them so they're round. Chill until ready to use.
Step 1Heat the oven to 375 degrees.
Step 2Melt the chocolate and butter in the top of a double boiler set over, but not touching, simmering water. Stir in the flour and remove from the heat to cool slightly.
Step 3Whip the yolks in the bowl of an electric mixer and slowly add the sugar until the whisk leaves a thick ribbon that stays on top of the batter when it is lifted out, about 5 minutes. Pour the chocolate into the yolk-sugar mixture and gently fold them together.
Step 4Spray 10 (1/2-cup) ramekins with cooking spray. Half-fill each with chocolate batter, place 1 ganache ball in the center and then continue filling to the top with more batter. Place the ramekins on a rimmed baking sheet. (The recipe can be made ahead to this point up to 1 hour in advance and stored at room temperature, or 1 week in advance and frozen, tightly covered.)
Step 5Bake the cakes until they begin to pull away from the sides of the ramekins, 13 to 15 minutes. Dust with powdered sugar.