Lightly spray ice cube tray with cooking spray and set aside.
In a double boiler over gentle heat, melt chopped chocolate. When smooth, stir in butter and 1/3 cup of chopped Oreos.
Spoon 2 tablespoons of chocolate into each tray opening, and tilt tray in all directions, allowing chocolate to cover the sides. Set aside for a couple of minutes to allow a candy shell to form, then refrigerate for 5 minutes.
In a small bowl, mix the chocolate buttercream and remaining chopped Oreos. Fill the tray by piping 1 1/2 teaspoons of chocolate Oreo buttercream in each recess. When finished, pour remaining chocolate over the entire tray. Knock the tray on the countertop to remove air bubbles, and refrigerate for 1 hour until set.