Level cakes with a serrated knife or cake leveler. On the top of TWO of the cake layers, use a piping bag to pipe frosting around the outside of the cake layers**. Fill the inside of each of the piped rings with 1/3 of the graham cracker mixture.
Turn oven to high broil. Line a cookie sheet with parchment paper. Place one of your 8" round baking pans on the parchment paper. Trace the bottom of the pan on the paper. Spray paper with cooking spray. In a single layer, spread 1 cup of mini marshmallows in the traced ring (leave about 1 inch between the outer marshmallows and the traced ring).
Place in oven and cook until marshmallows are golden brown. This takes about 2 to 3 minutes. Watch very closely as this happens quickly!
Place the browned marshmallows on top of the graham cracker crumbs on one of the cake layers. The sticky side of the marshmallows should be toward the graham crackers, and the browned side should be facing upward.
Repeat process with a clean piece of parchment paper and the remaining cup of mini marshmallows.
Stack the two filled cake layers on top of each other. Place the final cake layer, leveled side down, on top. Use frosting to frost outside and top of cake as desired (I use just a little so that the sides of the cake still peak through; see photos).
Again, turn oven to high broil. Line a cookie sheet with parchment paper and spray with cooking spray. Place large marshmallows on paper, an inch or two apart. Cook until marshmallows are golden brown. Again, this will take 2-3 minutes and you need to watch very closely.
Stack marshmallows on top of cake as desired.
Use remaining graham cracker crumbs to press into the sides of the bottom of the cake and sprinkle over the top of cake.