Preheat the oven to 350°F. Spray a 12-inch pizza pan with nonstick baking spray. For easy removal, line the bottom with a circle of parchment paper.
Prepare the brownie batter according to the package directions. Spread the batter evenly into the prepared pizza pan. Sprinkle the milk chocolate chips over the top, followed by the marshmallow bits.
Bake for 12 to 15 minutes. Since brownie mixes vary, you may need a few extra minutes. The brownie pizza is done when the center is set and a toothpick inserted into the middle comes out mostly clean.
Let the brownie pizza cool completely.
Meanwhile, melt the chocolate melting wafers according to the package directions. If you’re piping the frosting around the edge, drizzle the melted chocolate over the brownie and sprinkle with the crushed graham crackers. If you’re spreading the frosting over the entire top, frost the brownie first, then drizzle with the melted chocolate and finish with the graham crackers.
To make the frosting, beat the butter until smooth. Add the powdered sugar, S’mores Spread, vanilla, and 2 tablespoons of heavy cream. Beat until smooth and fluffy, adding the remaining tablespoon of cream if needed for your desired consistency.
Spread the frosting over the brownie, or transfer it to a piping bag fitted with a 1M tip and pipe a border around the edge.
Finish by decorating with mini marshmallows and mini chocolate chips. Slice and serve.