Preheat oven to 350°F and spray a 24-cup mini muffin tin with nonstick spray.

In a large bowl, beat butter and sugars until smooth. Add the egg and vanilla and mix well.

In another bowl, whisk together flour, graham cracker crumbs, baking soda, and salt.

Slowly add the dry ingredients to the wet mixture, mixing until just combined.

Press dough evenly into the 24 mini muffin cups. Bake for 8-10 minutes.

Remove from the oven and gently press down the centers with a teaspoon or similar tool. Halve the marshmallows and place one in each cookie cup.

Set the oven to broil and carefully toast the marshmallows until golden.

Place a small chocolate bar rectangle on each marshmallow. The warmth will melt the chocolate slightly.