Melt the butter. Measure out 4 tablespoons and set aside. Crush Oreos to fine crumbs. Remove 1/3 cup and set aside.
Mix remaining cookies with butter, and press into the bottom of a greased springform pan.
Freeze until set, about 30 minutes.
In the bowl of a stand mixer, beat cream cheese and 1/3 cup sugar until light and fluffy. Add cream and beat 2 minutes more. Fold in reserved cookie crumbs.
Transfer cream mixture to pan with cookie crumbs and smooth top. Refrigerate until firm.
Meanwhile, in a small saucepan, combine chocolate, evaporated milk, remaining 1/4 cup butter, and remaining 1/2 cup sugar. Bring to a low boil and cook for 4 minutes, stirring constantly. Cool to room temperature.
Pour over the surface of the cheesecake then return to refrigerator until set.
Remove from refrigerator and dust the top with cocoa powder.