½ cup/115 grams unsalted butter (1 stick), cut into chunks, plus more for greasing the pan
⅓ cup/30 grams unsweetened cocoa powder
1 ½ tablespoons cornstarch
¼ teaspoon baking powder
¼ teaspoon fine sea salt
5 ounces/140 grams semisweet or bittersweet chocolate, coarsely chopped
½ cup/100 grams granulated sugar
3 large eggs, chilled
For the ganache:
1 ¾ cups/420 milliliters heavy cream
6 ounces/170 grams semisweet or bittersweet chocolate, finely chopped
For the topping:
3 tablespoons unsweetened cocoa powder
Preheat oven to 325 degrees. Butter a 9-inch cake pan, line with parchment paper and butter the paper.
Sift together the cocoa powder, cornstarch, baking powder and salt in a medium bowl. Whisk to blend.
Put the 1/2 cup butter in a large heatproof bowl set over a saucepan of simmering water. Add the semisweet chocolate stirring often, until the mixture is smooth and glossy. Remove the bowl from the pan, and stir in the sugar. One by one, energetically stir in the eggs, beating for 1 minute after the last egg is added. The mixture will look like pudding. Stir in the dry ingredients. Scrape the mixture into the cake pan, and give the pan a couple of good raps against the counter to settle the batter.
Bake for 18 to 20 minutes, Transfer to a rack, cool for 5 minutes, then unmold the cake. Peel off the paper, invert the cake and cool to rack.
Make the ganache: Pour 1 1/4 cups cream into a small saucepan; refrigerate the rest. Scald the cream over medium heat, turn off the heat and stir in the semisweet or bittersweet chocolate until fully incorporated. Transfer to a heatproof bowl. Refrigerate the ganache for 10 minutes, whisk it, then refrigerate again for 10 minutes. Repeat chilling and whisking steps until the ganache is thick enough to make tracks when you stir, 50 to 60 minutes.
Carefully return the cake to mold, Whip the remaining 1/2 cup cream until it holds medium peaks.
Fold cream into ganache, refrigerate over night & dust with cocoa powder.