Make the Oreo Crust:

1. Preheat your oven to 350°F (175°C). Lightly grease a 9-inch pie pan with butter or cooking spray.
2. Crush the Oreos by pulsing them in a food processor until they form fine crumbs, or place them in a sealed plastic bag and crush with a rolling pin. You should have about 2 ½ cups of crumbs.
3. Combine the Oreo crumbs with 5 tablespoons of melted butter in a medium bowl, stirring until the mixture resembles wet sand and holds together when pressed.
4. Press the mixture firmly into the bottom and up the sides of your prepared pie pan, creating an even layer. Use the bottom of a measuring cup to pack it down tightly.
5. Bake the crust for 8 minutes, then remove and set aside to cool slightly while you prepare the brownie layer.

Prepare the Brownie Layer:

6. Melt the butter and chocolate together in a medium saucepan over low heat, stirring constantly until smooth and fully combined. Remove from heat and let cool for 5 minutes.
7. Whisk in both sugars until well incorporated and the mixture is glossy.
8. Add the eggs one at a time, whisking well after each addition until the mixture is smooth and thick.
9. Stir in the vanilla extract.
10. Combine the dry ingredients in a separate bowl: flour, cocoa powder, salt, and baking powder. Whisk together to remove any lumps.
11. Fold the dry ingredients into the chocolate mixture using a rubber spatula, stirring just until no flour streaks remain. Don’t overmix.
12. Pour the brownie batter into the prepared Oreo crust, spreading it evenly with a spatula.
13. Bake for 25-28 minutes until the top looks set but the center is still slightly soft. A toothpick inserted near the center should come out with a few moist crumbs.

Add the Marshmallow Layer:

14. Remove the pie from the oven and immediately sprinkle the mini marshmallows evenly over the hot brownie layer.
15. Return to the oven for 2-3 minutes, just until the marshmallows puff up and begin to soften. Don’t let them brown.
16. Remove from the oven and let cool to room temperature, about 1 hour. The marshmallows will deflate slightly as they cool.

Make the Chocolate Ganache:

17. Heat the heavy cream in a microwave-safe bowl for 60-90 seconds until it just begins to simmer (or heat in a small saucepan on the stovetop).
18. Pour the hot cream over the chocolate chips in a heatproof bowl. Let sit undisturbed for 2 minutes to allow the chocolate to melt.
19. Add the butter, vanilla, and pinch of salt, then whisk gently until the mixture is completely smooth and glossy.
20. Let the ganache cool for 10-15 minutes until it thickens slightly but is still pourable.
21. Pour the ganache over the marshmallow layer, starting from the center and working outward. Use an offset spatula to spread it evenly to the edges, covering all the marshmallows.
22. Refrigerate the pie for at least 2-3 hours, or until the ganache is completely set and firm.
23. Slice and serve chilled or at room temperature. Use a sharp knife dipped in hot water and wiped clean between cuts for the neatest slices.