How to Make It
Combine butter, oil, and chocolate in the top of a double boiler. Cook over simmering water until chocolate almost fully melts, stirring gently with a spatula. Remove top of double boiler; stir until chocolate fully melts.
Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cocoa, baking powder, and salt in a bowl; stir well with a whisk.
Place granulated sugar, brown sugar, vanilla, and eggs in a large bowl; beat with a mixer at medium speed until light and fluffy (about 2 minutes). Set aside 2 1/2 tablespoons chocolate mixture. Gradually pour remaining chocolate mixture in a thin stream over egg mixture, beating at medium speed. Gently fold flour mixture into egg mixture. Divide batter evenly among 10 (5-ounce) ramekins coated with baking spray. Working with 1 ramekin at a time, spoon 3/4 teaspoon reserved chocolate mixture into center, pushing teaspoon toward center of batter. Repeat with remaining ramekins and chocolate mixture. Arrange ramekins on a jelly-roll pan. Cover and refrigerate 1 hour.
Preheat oven to 400°.
Let ramekins stand at room temperature 15 minutes. Uncover and bake at 400° for 13 minutes or until cakes are puffy and slightly crusty on top (centers will not be set). Place a dessert plate on top of ramekin. Using a dry kitchen towel to steady ramekin, invert each cake onto plate. Garnish with powdered sugar, if desired. Serve immediately.