Fold a 12-inch square of aluminum foil into quarters to make it 4-ply. Wrap it around a 3-inch-long stick of butter, leaving the top open. Slip the butter out and set aside the mold. In a small bowl, microwave the chocolate and oil at medium power, stirring halfway through, until melted and smooth, about 2 minutes. Pour into the mold, tapping to level the surface. Refrigerate until firm, about 6 hours.
Unwrap and let stand at room temperature for 10 minutes before use. Use a vegetable peeler to make curls, a serrated knife to make shavings, and a Microplane zester to make dust.