Preheat oven to 325°F (170°C). Spray 2 (8-inch) round cake pans with baking spray with flour.

In a large bowl, whisk together flour, baking powder, and salt.

In a medium bowl, whisk together hot coffee and cocoa until smooth. Whisk in milk. Add brown sugar, oil, egg, and vanilla, and whisk until combined. Gradually add flour mixture, whisking just until combined. Divide batter between prepared pans (about 300 grams each). (Batter will be thin.)

Bake until cake pulls away from sides of pan and a wooden pick inserted in center comes out clean, about 15 minutes. Let cool completely in pan.

Level cooled cake layers, if desired. Place 1 cake layer in center on base of a 9-inch springform pan without sides.

Place White Chocolate Filling in a large pastry bag, and cut a 1-inch opening in tip. Pipe an even layer on top of cake layer in pan, smoothing with an offset spatula. Top with remaining cake layer. (Make sure filling is smooth and does not stick out past sides of cake.) Place sides on springform pan; cover and refrigerate while preparing mousse.

Place Chocolate Mousse in a large pastry bag, in batches as needed, and cut a 1-inch opening in tip. Pipe around outside edges and on top of cake, smoothing with an offset spatula. (Springform pan will be full and mousse will come to the top). Cover and freeze until firm, about 4 hours or up to overnight.

After preparing the Chocolate Mirror Glaze and as it is cooling, using a small kitchen torch or a hot towel, heat sides of springform pan. Remove sides of pan, and using a large spatula, transfer cake to a 9-inch cake board. Using a hot, dry spatula, smooth edges and top as needed. Freeze until glaze reaches correct temperature.

Line a rimmed baking sheet with plastic wrap; place a wire rack on top. Place cake on prepared rack. Pour Chocolate Mirror Glaze over cake, making sure sides are completely covered. Using a large spatula, swipe top of cake once to remove any excess glaze. (Do not do this more than once; it will start to set and will not be completely smooth anymore.) Refrigerate until glaze is set and cake has thawed in center, about 3 hours. Serve cold.