Mix butter with both sugars until smooth in a large mixing bowl using a wooden spoon or electric mixer.
Add egg, vanilla, butter extract, baking soda and salt and mix until smooth, then mix in flour.
Unwrap 2 Hershey Bars and break apart into pieces, then break the small rectangles into 3-4 pieces each.
Stir the graham crackers, broken Hershey Bars, and ½ cup mini marshmallows into the cookie dough.
Line a cookie sheet with parchment paper or silicone baking mat. Scoop 2-tablespoon size cookie dough balls onto baking sheet. Crowding doesn’t matter, we are chilling the dough. Chill for at least 30 minutes.
Important note: make sure the marshmallows are inside the cookie dough, not sticking out the bottom or sides where they will come in contact with the cookie sheet. Push and roll the dough balls as needed to seal the marshmallows inside (showing on top is okay). Any that come in contact with the cookie sheet will melt while baking.
Preheat oven to 350°F. Line a second cookie sheet with parchment paper or a silicone baking mat.
Spread chilled cookie dough balls at least 2-inches apart on prepared cookie sheets. Bake 11 minutes. While the cookies are baking, break apart remaining Hershey Bars (each rectangle into 2 pieces) and measure out about 1/3 cup more mini marshmallows.
After the cookies are partially baked at 11 minutes, remove from oven and quickly press 2-3 candy bar pieces and 2-3 mini marshmallows on the top of each one. Return to the oven to finish baking, an additional 3-4 minutes, or until they lose their glossy sheen.
Cool 5 minutes on pan before removing to a rack to cool completely.
Store in an airtight container for up to 3 days or freeze for up to 3 months.