Set to 350°F (175°C) and line a baking sheet with parchment paper or a silicone mat.
In a large bowl, beat softened butter, granulated sugar, and brown sugar with a mixer for 2–3 minutes until light and fluffy.
Mix in the egg and vanilla extract until fully combined.
In a separate bowl, whisk together flour, baking soda, and salt. Gradually add to the butter mixture and mix until just combined.
Stir in the crumbs until the dough holds together and has a slightly sandy texture.
Roll tablespoon-sized portions into balls and place them 2 inches apart on the baking sheet.
Bake for 8–10 minutes, until edges are golden but centers are still soft.
Right out of the oven, press 3–4 mini marshmallows and a few chocolate chips into the center of each cookie.
After 1–2 minutes, press a Hershey’s Kiss into the soft center. Let cookies rest on the baking sheet for 5 minutes, then move to a wire rack to cool.