For peanut butter fudge filling: Prepare 2 round cake pans by lining with parchment and spraying with pan spray. In a mixer, combine all ingredients except the cream. Mix on low speed until combined and add in cream. The mixture may be slightly crumbly, but should hold shape when pressed together. Split peanut butter filling between the two already prepared pans. Place in freezer to help solidify for about 15 minutes. Once the mixture is cool and has hardened slightly, turn pan upside down and bang out of the pan.
For brownies: Preheat oven to 350 degrees. Spray 3 pans with baking spray. Whisk eggs, add in water and oil. Stir in mix and combine until smooth. Divide into three pans and bake for 18 minutes, or until toothpick comes out clean. Allow brownies to cool and remove from pans.
To assemble cake: Start with a brownie layer, then place a peanut butter layer on top, brownie layer, peanut butter layer and final brownie layer. To serve, use a hot knife to cut through layers. The filling is rich, so this cake goes a long way!