Preheat the oven to 325 degrees F. Spray a cake pan with cooking spray and/or line with parchment paper.
For the Brownie Layer: In a medium-sized bowl, add the protein powder, flour, black cocoa, sweetener, baking powder and baking soda. Combine with a whisk until there are no clumps. Add the pumpkin, butter, milk, and melted chocolate. Using a hand mixer (or stand), mix to combine. Pour the batter into a prepared dish. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out mostly clean.
For the Peanut Butter Frosting: Add all ingredients to a medium-sized bowl. Using a hand mixer, blend until smooth.
Once the brownie cake has cooled, transfer it to a cake plate (or other plate that you can easily store in the fridge). Spread the peanut butter frosting evenly onto the middle of the cake, leaving a 1/2 to 1-inch of brownie later showing.
For the Chocolate Topping: Add chocolate chips to a microwave-safe bowl. Microwave in 30-second intervals, stirring in between, until chips are melted. Let melted chocolate cool for 5-10 minutes, then add the chocolate on top of the peanut butter frosting layer. Cover the cake (with a cake topper or another bowl to create a dome) and place in the fridge immediately to allow chocolate to set.
To make Low Fat Buckeyes: In a medium mixing bowl, add the peanut butter powder, powdered sugar and salt. Whisk to combine the dry ingredients. Add the water, then mix with a wooden spoon or hand mixer until all ingredients are combined. Note the batter will be very thick.
Use a cookie scoop to scoop out 16 portions (about 2 Tbsp, or 29g each) and place them on a baking sheet lined with waxed paper. Place the sheet into the freezer for about 30 minutes.
Melt the chocolate in the microwave, stirring every 30 seconds until you get a smooth, shiny consistency. Let the chocolate cool but not harden.
Remove the peanut butter balls from the freezer and roll them into smooth balls with your hands.
Dip each ball in the melted chocolate, transfer them back to the waxed paper and place them in the refrigerator to set. Keep refrigerated until ready to serve.
When ready to serve, add Buckeyes on top of Brownie Cake and enjoy!