Preheat oven to 375 degrees. Prepare foil cake pan with nonstick spray spray. Cut butter into 1/2" pieces (doesn't have to be perfect). Set up a double boiler (small pot with 2" simmering water with a heat-safe bowl on top.) In the double boiler, melt butter and 2/3 cup chocolate.

Once butter and chocolate are totally melted, remove from heat and whisk in sugar until smooth. Add egg and whisk to combine. Add all but 1/2 tsp. cocoa powder (reserving 1/2 tsp. for garnish) and whisk just until it's incorporated.

Fill prepared cake pan 3/4 full with batter and bake about 25 minutes. When cake is completely cooked, a skin will have formed on the top and there will be some small cracks around the edges. The whole cake will puff up just a little bit (similar to a soufflé). The middle of the cake will should be set but the entire cake will have a slight jiggle to it. Let cool completely before un-molding. If you would like to speed this process up, put it in the refrigerator, uncovered, until you can't feel any warmth when placing your hand on the bottom of the pan.

When cooled, un-mold the cake. It is easiest to pull the sides of the foil pan away from the cake and then flipping it onto a plate. Set un-molded cake aside; if you find it is still slightly warm, you can put it back in the refrigerator while you make the ganache. Put remaining chocolate in a small heat-safe bowl and set aside. In a small pot, heat heavy cream to a simmer, then pour it over chocolate. Let it sit 1 minute and then stir until the chocolate is fully melted, shiny, and smooth. (Use a rubber spatula or wooden spoon. Do not whisk: it will make the ganache full of bubbles.) This can be gently heated if chocolate does not completely melt from hot heavy cream; either over a low flame or by 20 second intervals in a microwave.

Place top of cake face down on a dinner plate. Pour warm ganache over chocolate cake starting in the center of the cake and then slowly circling out towards the edge until it spills over the sides and completely covers the cake. If there are air bubbles, or if you need to level it out, simply tap the plate on your counter and they should pop; if not, poke with a knife or fork. If you'd like to transfer it to a clean plate, simply slide a spatula or a knife underneath the cake and carefully pick it up to move it.

Sprinkle on reserved cocoa powder, cut and serve! (Use a warm, clean knife to make clean cuts in your cake.)