1. Prepare your brownies according to the directions on the box or mix. Bake into a 8, 9 or 10 inch springform pan. Allow to cool completely and leave in pan.
2. To prepare peanut butter crunch layer, melt your butter and chocolate chips in a heavy saucepan on low (this can also be done in the microwave - just go low and slow with the heat and stir often). Once melted, remove from heat and stir in peanut butter and crushed corn flakes. Mix well to combine. Once combined, spread on top of brownies.
3. To prepare mousse layer, melt chocolate, milk and marshmallows in a heavy saucepan on low. Once melted, remove from heat and allow to cool completely (approx 30 minutes, but give it a stir every now and then). Pour whipping cream into a chilled mixing bowl and beat on high until stiff peaks form. Gently fold melted chocolate, milk and marshmallow mixture into your whipped cream, stirring until combined. Spread on top of peanut butter crunch layer.
4. Place in refrigerator to set for at least three hours.
5. When ready to serve, remove sides from springform pan, slice and serve. If desired garnish with peanut butter frosting, whipped cream and/or peanut butter cups.