Heat oven to 350°F. Line an 8" (20 cm) round cake pan with parchment paper, set aside.
In a bowl of a mixer mix together the dry ingredients; 2/3 cup flour, 1/3 cup cocoa powder, 2/3 cup sugar, 3/4 tsp baking soda, 1/2 tsp baking powder, and a dash of salt. Add the wet ingredients; 1 egg, 1/4 cup milk, 3 Tbsp oil, 1 tsp vanilla extract, 1⅓ cup water and mix slowly until the batter is smooth. It will be very runny, but that's how it is supposed to be.
Soak 2¼ cup gelatin in 1/4 cup water.
Whip 2 egg yolks with 1/8 tsp salt and 2 Tbsp corn starch. Bring milk to a boil, add 2 tsp instant coffee and stir until it dissolves. While continuously whisking the egg yolks, slowly add the hot milk. Now transfer the egg and milk mixture back to the pot and cook it until it thickens, constantly stirring to prevent the bottom from scorching.
Meanwhile combine 2 tbsp water & 1/3 cups sugar in a saucepot over high heat, mixing to dissolve the sugar. Once boiling, stop stirring and continue cooking for 2 minutes.
Heat 3/4 cup heavy whipping cream with 1 Tbsp corn starch, pour it over the chocolate, allow to sit for 2-3 minutes, stir until smooth. Now add 1 tsp vanilla extract and stir until incorporated. Pour the ganache over the chilled mousse cake and allow to set in the fridge.