Step 1: Preheat oven to 325°F and line an 8×8 inch baking pan with parchment paper.
Step 2: Mix graham cracker crumbs with melted butter until combined, then press firmly into the bottom of the pan.
Step 3: In a large bowl, beat cream cheese and sugar until smooth.
Step 4: Add eggs one at a time, beating after each addition.
Step 5: Mix in sour cream, vanilla extract, and cocoa powder until fully incorporated.
Step 6: Pour cheesecake mixture over the crust and spread evenly.
Step 7: Bake for 30-35 minutes until edges are set but center is slightly jiggly.
Step 8: Remove from oven and sprinkle mini marshmallows over the top.
Step 9: Return to oven for 3-5 minutes until marshmallows are lightly toasted.
Step 10: Cool completely at room temperature, then refrigerate for at least 4 hours.
Step 11: Melt chocolate chips and drizzle over bars before cutting into squares.