Spray a microwave-safe mug with cooking spray; set aside. Place flour, 1 tablespoon graham cracker crumbs, sugar, baking powder, salt, milk, egg and vanilla in a small mixing bowl and stir until well combined. Pour batter into the prepared the mug.
Add milk, stirring until no clumps remain. Use a fork or whisk to gently beat the egg into the batter, making sure that it is fully incorporated. Sprinkle with chocolate chunks, marshmallow.
Microwave on high for about 2 minutes, depending on microwave strength and depth of mug.
Remove from microwave, grab a spoon and enjoy! If desired top cake with additional chocolate and a sprinkle of of graham cracker crumbs.