Baked Chocolate Mousse Cake

Preheat the oven to 325 degrees F (165 degrees C).

Mix boiling water and cocoa powder for cake together in a small mixing bowl until smooth. Let cool, about 10 minutes.

Meanwhile, sift flour, sugar, baking soda, and salt together in a large bowl. Make a well in the center, pour in oil, egg yolks, vanilla extract, and cooled cocoa mixture. Mix using a wooden spoon just until smooth.

Sprinkle cream of tartar over egg whites in a large mixing bowl. Beat using an electric mixer on high speed until very stiff peaks form. Pour cake batter over egg white mixture and fold gently using a whisk with an under-and-over motion; be sure not to overmix, but to also fully incorporate ingredients. Pour batter into an ungreased 10-inch tube pan.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 1 hour. Let cake cool in the pan for 1 1/2 hours. Loosen with a spatula and transfer to a plate.

Add cold water to a glass bowl and sprinkle gelatin on top; allow to soften. Place bowl over a saucepan filled with simmering water. Heat and stir until gelatin is fully dissolved, 2 to 3 minutes. Remove from heat and let cool.

Combine heavy cream, confectioners' sugar, cocoa, vanilla extract, and salt in a large bowl. Refrigerate for 1 hour. Beat mixture using an electric mixer until stiff; keep mousse in refrigerator until needed.

Cut a 1-inch slice crosswise from the top of the cooled cake and set aside. Outline a cavity in the cake using a sharp knife, leaving 1-inch walls around cake side and bottom. Carefully remove cake from the cavity using a spoon; reserve.

Measure 2 1/2 cups of mousse filling and fold in gelatin. Fill the cake cavity and replace top of cake. Mix 1/2 cup filling with cake crumbs and fill center hole of cake. Frost the top and sides with remaining filling. Refrigerate until set, 2 to 3 hours.