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
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.