Position racks in upper and lower positions of oven. Preheat oven to 350°F/180°C. Line three rimmed half-sheet baking pans with parchment paper; set aside.

In a small bowl, whisk together the flour, graham cracker crumbs, baking soda and salt; set aside.

Cream the softened butter in a mixing bowl with an electric mixer, then add brown sugar and sugar and beat on high until fluffy. Beat in vanilla, then egg and beat until combined. Beat in dry mixture until a few floury streaks remain, then beat in chocolate chunks and marshmallow bits until evenly blended.

Dole out evenly spaced on pans in about Ping Pong sized balls; no need to flatten. Bake two pans first for about 10 to 12 minutes, one on each rack, rotating front to back and top to bottom halfway through baking. When done the cookies will darken and flatten and will be firmer around the edges and still a tad soft in the center; they will firm up upon cooling. Cool on racks and bake the last pan. Don’t try and remove cookies from pans until they cool. Store in single layers, separated by parchment paper, in airtight containers for up to 4 days.