01 - Preheat the oven to 350°F. Grease and line a 9-inch round cake pan with parchment paper.
02 - Sift together the flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl. Set aside.
03 - Melt the chopped bittersweet chocolate in a heatproof bowl set over simmering water, stirring until smooth. Allow to cool slightly.
04 - Using an electric mixer, beat the softened butter and granulated sugar together until light and fluffy, approximately 3 minutes.
05 - Add eggs one at a time, beating well after each addition. Mix in the vanilla extract thoroughly.
06 - Fold the melted chocolate into the wet mixture until just combined.
07 - Add half of the dry ingredients to the wet mixture, mixing on low speed. Pour in the milk and combine. Add remaining dry ingredients and mix until just incorporated.
08 - Gradually add the hot coffee, stirring gently until the batter is smooth and glossy. The batter will be thin.
09 - Pour the batter into the prepared pan and smooth the surface evenly.
10 - Bake for 33 to 36 minutes, until a toothpick inserted into the center comes out with a few moist crumbs.
11 - Allow the cake to cool in the pan for 10 minutes before turning out onto a wire rack to cool completely.