Crush the Oreos into crumbs and mix with ¾ cup of melted butter. Press this into a 9x13 greased pan and bake for 8 minutes at 350°F.
Make the filling by melting 1 cup of butter and mixing it with ½ cup of cocoa. Let it cool a bit.
Beat 4 eggs, then whisk in 1 ¾ cups of sugar, a pinch of salt, and vanilla. Fold in 1 ½ cups of flour and 2 cups of pecans.
Spread the filling over the baked crust. Bake at 350°F for 40 minutes, then sprinkle 3 cups of mini marshmallows on top for the last 5 minutes.
Make the topping by whisking together 4 cups of powdered sugar, ⅓ cup of cocoa, ⅔ cup of evaporated milk, and ½ cup of melted butter. Spread this over the cooled dessert.
