Heat oven to 350°F (175°C). Whisk all dry cake ingredients together, whisk all wet ingredients separately including the hot coffee, combine, divide between 2 greased and lined 9-inch pans, and bake for 32 to 35 minutes until a toothpick comes out with a few moist crumbs. Cool completely in the pans.

Bloom the gelatin in the cold water for 5 minutes. Heat 1/2 cup heavy cream until steaming, pour over the chopped white chocolate, stir smooth, dissolve in the gelatin, and cool to room temperature. Whip the 1 1/2 cups cold heavy cream to stiff peaks, fold into the cooled white chocolate base in 3 additions, then refrigerate for 20 minutes.

Place one cake layer on a board, spread all of the white chocolate mousse on top, set the second layer on, and refrigerate for 30 minutes.

Chop the dark chocolate and place in a bowl. Heat the 3/4 cup heavy cream to a simmer, pour over the chocolate, rest 2 minutes, stir smooth, then stir in the 2 tbsp butter. Cool for 10 to 15 minutes.

Pour the ganache over the center of the chilled cake, spread to the edges, allow drips to fall naturally, and refrigerate for at least 1 hour. Slice with a hot, dry knife and serve with the white mousse layer visible in each cross-section.