First Step: Prepare baking equipment and preheat oven Preheat your oven to 350°F (177°C). Grease a 9-inch springform pan and line the bottom with parchment paper. Secure the pan ring for easy removal after baking.

Second Step: Mix dry ingredients In a medium bowl, whisk together all-purpose flour, cornstarch, Dutch cocoa powder, baking soda, baking powder, and kosher salt.

Third Step: Combine wet ingredients and sugars In a separate large bowl, beat granulated sugar, brown sugar, and vegetable oil until the mixture resembles wet sand. Add eggs one at a time, then stir in vanilla extract and sour cream to create a smooth batter.

Fourth Step: Incorporate dry ingredients and coffee Add half the dry mixture to the wet batter, mix gently, then pour in hot coffee or water. Stir until the batter is smooth, then fold in remaining dry ingredients.

Fifth Step: Bake and cool the cake Pour batter into the prepared pan and bake for 25-30 minutes, or until a toothpick comes out with moist crumbs. Cool the cake completely on a wire rack. Level the top, reattach the pan ring, and line sides with acetate sheets for a neat mousse border.

Sixth Step: Soak the cake Mix hot coffee with cocoa powder until dissolved. Evenly spoon this soak over the cake to enhance moistness and chocolate flavor.

Seventh Step: Make the chocolate mousse Melt dark and milk chocolates over a double boiler and allow to cool slightly. Whisk egg yolks into the melted chocolate one at a time. Bloom gelatin in water. Warm half the heavy cream until steaming, whisk in gelatin until dissolved, then combine gradually with the chocolate mixture. Fold in the remaining chilled cream gently.

Eighth Step: Prepare meringue and combine with mousse Whip egg whites and cream of tartar to frothy peaks, gradually add sugar, and whip to soft peaks. Gently fold whipped cream into chocolate mixture, then fold in meringue for a light, airy mousse.

Ninth Step: Assemble and chill Pour mousse over soaked cake in the pan lined with acetate collar. Refrigerate at least 4 hours or overnight for proper set.

Final Step: Prepare ganache and finish cake Heat heavy cream until just hot (do not boil). Pour over chopped dark chocolate, sit for 3-5 minutes, whisk until smooth. Remove acetate and pan ring, pour ganache on mousse layer, letting it drip down sides. Chill 30 minutes until ganache sets. Garnish as desired.