Preheat the oven to 350F. Line a baking sheet with parchment paper or ungreased baking sheet.
Cream together the butter, peanut butter, granulated and brown sugar.
After it is creamy, add in the egg, vanilla extract and salt and beat to combine.
Blend in the graham cracker crumbs, flour, soda, and salt in and mix until just combined.
Scoop cookies onto baking sheet using a large cookie scoop (¼ cup) or medium cookie scoop (1 ½ to 2 tablespoons) or roll into balls and slightly flatten.
For large cookies, bake 7 minutes, then top each with marshmallow half and bake 5 minutes more.
For medium sized cookies, bake 5 minutes, then top each with marshmallow half and bake 4 minutes more.
Let cookies rest on baking sheet for 5 minutes before removing. They have to set up and transferring them too soon, they will break apart.
After a few minutes, press a piece of chocolate bar on top gently into the marshmallow.
Allow to cool on a wire rack, then enjoy!