Combine cake mix according to the ingredients on the box. Beat on medium speed until well combined.
Bake cake in a 9x13-inch pan for 18-22 minutes. Check for doneness by inserting a toothpick into the middle of the cake. If the toothpick comes out clean, you cake is done. Allow to cool for at least 30 minutes.
While the cake is still warm, take the end of a wooden spoon or another round object and poke holes all over the top of your cake.
For the filling: In a small mixing bowl, combine the chocolate pudding and milk and whisk until the pudding mix is dissolved. Before the pudding sets (do not refrigerate), pour it over top of the cake, and spread with a spatula to fill in all the holes.
Refrigerate the cake for at least 15-30 minutes until the pudding is set and the cake is completely cooled.
For the topping: Put the bowl and whisk attachment in the freezer for 5-10 minutes to get them really cold (this step is optional, but a common practice I use).
Place 1 tablespoon of cold water in a small dish. Sprinkle the gelatin over the cold water, trying to spread the powder evenly. Allow the gelatin to turn to a solid (about 2-3 minutes).
Next, melt the solid gelatin in the microwave for 5-8 seconds until it turns back to a liquid. Watch it closely!
Pour the heavy whipping cream into the chilled bowl and beat on medium speed for about 3 minutes until the cream starts to thicken.
Slowly add the liquid gelatin and continue beating until soft peak form.
Turn the mixer down to low speed and slowly add the dry brownie mix. Once the dry mix is incorporated, turn the mixer back to a higher speed and beak until stiff peaks form.
Chop the prepared brownies into small pieces, about the size of a dime or smaller and gently fold them into the brownie batter mousse. Spread the mousse over the cooled cake.
This cake must be refrigerated. Garnish with hot fudge sauce and more crumbled brownie pieces.
Recipe notes: For the actual brownies in the mousse, I tend to use store bought brownies and chop them up nice and small, about the size of a dime. Of course, it never hurts to make a batch for brownies at home, but it does add some steps to the process. So, it’s your choice!