CAKE: In a large mixing bowl, beat together the sugar, butter, salt and flavorings until fluffy. Beat in the oil, then the eggs, one at a time, beating till the mixture is very fluffy. In a separate bowl, whisk together the flour, Instant ClearJel(r) and baking powder. Add the dry ingredients to the butter and egg mixture alternately with the milk, beating until well-combined, about 2 minutes.
Lightly grease and flour both halves of the filled cake set, and divide the batter between them. (You may also prepare this cake in two regular 9 x 2-inch round cake pans.) Bake the cake in a preheated 350°F oven for 40 to 45 minutes, until the edges have started to pull away from the sides of the pan. Cool the cake for 15 minutes before removing it from the pans.
Filling: Follow the directions on the package to prepare the mousse. Divide the mousse between the two halves of the cake, then invert the top half over the bottom half (you're forming a tunnel for the filling here).
Glaze: In a small bowl in the microwave, or a small saucepan set over low heat, prepare the glaze by melting together the cream and chocolate. Drizzle it over the cake.