Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

In a large bowl, use an electric mixer to cream together the butter, peanut butter, granulated sugar, and brown sugar until light and fluffy, about 2–3 minutes.

Add eggs one at a time, mixing well after each. Stir in vanilla extract until fully incorporated.

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

Gradually add the dry ingredients to the wet mixture, mixing on low speed just until combined. Do not overmix.

Scoop dough into 1-inch balls (about 1 tbsp each) and place on baking sheets 2 inches apart. Slightly flatten each with your palm or a glass bottom.

Bake cookies for 10–12 minutes, until edges are golden but centers are slightly soft. Let cool on sheet for 5 minutes before transferring to a wire rack.

In a microwave-safe bowl, melt chocolate chips and peanut butter in 20-second intervals, stirring between each until smooth. Stir in marshmallows and let sit 5–10 minutes to thicken.

Turn half the cookies upside down. Spread 1–2 teaspoons of filling on each, then top with another cookie to form sandwiches.

Let cookies sit for 30 minutes at room temperature (or refrigerate briefly) to allow the filling to firm before serving.