Place a rack in the middle of the oven and preheat it to 325 degrees. Prepare a 9-inch pan by buttering it, lining it with parchment paper, and buttering that paper as well.

In a medium bowl, combine the cornstarch, baking powder, and cocoa powder. Blend the ingredients together.

In a heatproof bowl, add 1/2 cup butter and set over a pan of simmering water. Stir the chocolate over the mixture often to ensure that it is smooth and glossy

Add the sugar to the bowl after removing it from the pan. Add the eggs one by one, stirring energetically after each addition for one minute. You will see a pudding-like mixture. Mix in the dry ingredients. The mixture should be scrapped into the pan, and the pan should be mashed against the counter a couple of times to settle the batter.

You should bake it for 18-20 minutes, or until a tester inserted in the center comes out clean. After 5 minutes, transfer the cake to a rack and unmold it. Invert the cake, peel off the paper, and cool to room temperature. Make sure your cake pan is clean and dry.

One and one-quarter cups of cream should be placed in a small saucepan. All remaining food should be refrigerated. Stir the semisweet or bittersweet chocolate into the cream after it has been scalded over medium heat until fully incorporated. Then, transfer to a heatproof bowl.

Then, whisk the ganache and then refrigerate it once again for 10 minutes. After chilling and whisking the ganache for 50 to 60 minutes, it should form tracks when stirred.

Make two crisscross patterns in the cake pan with two pieces of parchment paper or foil that are 3 by 16 inches. Inspect the pan carefully before returning the cake.

In a large bowl, whisk 1/2 cup of cream until medium peaks form.

With a whisk, beat the ganache until it becomes soft and spreadable. Fold the whipped cream in with a spatula. Refrigerate for 2 hours after spreading over the cake. You can serve the cake at room temperature or cool, so remove it from the refrigerator about 20 minutes before serving.

Finally, shake the cocoa powder over the cake with a fine-mesh strainer. Take a table knife and run it along the sides of the pan. Carefully remove the cake from the pan and place it on a serving plate with parchment paper or foil handles.

Remove the strips and discard them. After rinsing the knife under warm water and wiping it dry between cuts, cut the cake into four equal portions.