Preheat the oven to 350°F and line two large baking sheets with parchment paper.

In a large mixing bowl, beat together the softened butter, peanut butter, granulated sugar, and brown sugar for 2 to 3 minutes until light and fluffy.

Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.

In a separate bowl, whisk together the flour, graham cracker crumbs, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture in three additions, mixing just until combined and a soft dough forms.

Scoop the dough into 1-inch balls (about one heaping tablespoon each) and place them on the prepared baking sheets, leaving about 2 inches between each cookie. Flatten each dough ball slightly.

Bake for 10 to 12 minutes until the edges are set and lightly golden while the centers remain slightly soft.

Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.

While the cookies cool, melt the chocolate chips and peanut butter together in a microwave-safe bowl in 20-second intervals, stirring between each interval until smooth.

Let the mixture cool slightly, then stir in the mini marshmallows. Allow it to sit for 5 to 10 minutes until it thickens to a spreadable consistency.

Turn half of the cooled cookies upside down and add a generous teaspoon or two of filling to each one.

Top with another cookie and gently press to form sandwich cookies.

Let the assembled cookies rest at room temperature for about 30 minutes to allow the filling to set.