Start by lining your 9x9 inch baking dish with parchment paper. This will make it much easier to lift the brownies out once they have set.
In a microwave-safe bowl, combine the 3/4 cup of unsalted butter and 1/4 cup of creamy peanut butter. Heat in the microwave in 30-second intervals, stirring in between, until fully melted and smooth.
In a large mixing bowl, combine the melted butter and peanut butter mixture with 1 cup of granulated sugar. Stir until well combined. Then, add the 2 slightly beaten eggs and 1 teaspoon of vanilla extract, mixing until smooth.
Gradually mix in the 2 1/2 cups of graham cracker crumbs and 1 cup of semi-sweet chocolate chips until everything is well combined. The mixture will be thick!
Gently fold in the 2 cups of miniature marshmallows into the brownie mixture until evenly distributed.
Transfer the brownie mixture into the prepared baking dish. Use a spatula to spread it evenly and press it down firmly.
In a microwave-safe bowl, melt the remaining 1/4 cup of semi-sweet chocolate chips. Once melted, drizzle this chocolate over the top of the brownie mixture.
Melt the remaining 1/4 cup of peanut butter (if needed) and drizzle it over the top of the brownies for an extra touch of flavor.
Refrigerate the brownies for at least 2 hours, or until they are firm enough to cut into squares.
