This is an easy one-bowl recipe for rich chocolate cake made with oil instead of butter. Using brown sugar and rice oil, mix together cake flour, cocoa powder, and baking powder, then evenly distribute the batter into muffin cups using a piping bag. Top with high-cocoa chocolate and bake in the oven. It becomes even more moist and delicious the next day. The size is perfect for gifts, and when warmed, the chocolate inside melts, making it an exquisite treat.