Preheat oven to 325°F (163°C). Line a 9×13-inch baking pan with parchment paper.

In a bowl, combine graham cracker crumbs, melted butter, and sugar. Press evenly into the bottom of the pan. Bake for 8 minutes. Let cool.

In a large bowl, beat the cream cheese until smooth. Add powdered sugar, sour cream, cocoa powder, and vanilla. Mix until creamy.

Add eggs one at a time, mixing just until incorporated. Fold in melted chocolate.

Pour the cheesecake batter over the crust. Smooth the top. Bake for 35 minutes or until the center is set but slightly jiggly.

Let cool to room temperature, then chill for at least 4 hours or overnight.

Once chilled, scatter mini marshmallows on top. Broil for 1–2 minutes until lightly toasted.

Chill briefly before slicing into 24 bars.