Preheat your oven to 350°F (175°C) and generously grease an 8x8 inch baking dish to ensure easy removal of the bars.
In a mixing bowl, combine 1 1/2 cups graham cracker crumbs, 1/2 cup melted unsalted butter, and 1/4 cup granulated sugar. Stir together until the mixture resembles wet sand.
Press the graham cracker mixture evenly into the bottom of the greased baking dish, creating a solid crust. Place this in the oven and bake for 10 minutes until lightly golden. This step will help firm up the crust.
After baking, remove the crust and allow it to cool for a few minutes. Once slightly cooled, add a layer of 1 cup semi-sweet chocolate chips over the crust, followed by 1 cup mini marshmallows, and 1/2 cup broken milk chocolate bars. Spread the chocolates and marshmallows evenly for consistent melty goodness.
In a small saucepan over low heat, melt 1/2 cup chocolate chips (for topping) until smooth. Drizzle the melted chocolate uniformly over the layered chocolate and marshmallows, which will create a delicious topping.
Return the dish to the oven and bake for an additional 5-7 minutes, or until the mini marshmallows turn golden brown and the chocolate is bubbly. Keep a close eye to avoid burning the marshmallows.
Once baked, remove from the oven and allow the dessert to cool completely in the dish. Once cool, cut into squares for serving.