Preheat your oven to 350°F (175°C). Grease a 9x13 baking dish with non-stick cooking spray or line it with parchment paper for easy removal

In a mixing bowl, combine the graham cracker crumbs, melted butter, and powdered sugar. Stir until the mixture resembles wet sand and is well combined. Firmly press the graham cracker mixture firmly into the bottom of the baking pan to create an even layer. You might find it helpful to use a spoon or a measuring cup to pack the crust down. Place in the oven and bake for 6-8 minutes until golden brown. The crust will firm up once it cools down.

Place the Hershey chocolate bars evenly over the graham cracker crust. Bake for 2-3 minutes until slightly melted. Remove from the oven and smooth the chocolate.

Scatter the mini marshmallows over the chocolate layer. Broil on low for 1-2 minutes until marshmallows are browned. Be sure to keep an eye on them while they broil so that they do not burn.

Cool and Slice Allow the bars to cool in the pan for about 15-20 minutes. Once cooled, use a sharp knife to cut the bars into squares or rectangles, depending on your preference.