1/2 cup plus 2 tablespoons flour
2 1/2 tablespoons cocoa powder
3/4 teaspoon baking soda
1/4 teaspoon salt
1/2 cup granulated sugar
1/2 cup water
3 tablespoons vegetable oil
1 1/2 teaspoons distilled white vinegar
1 teaspoon vanilla extract
1 cup heavy whipping cream
6 ounces finely chopped white chocolate
3 ounces finely chopped bittersweet chocolate
3 tablespoons boiling water
1 tablespoon egg white, at room temperature
1 pinch cream of tartar
1/2 cup plus 2 to 3 tablespoons confectioners’ sugar
Cupcakes: Position a rack in the center and preheat oven to 325 degrees F. Lightly butter 24 (1 3/4 x 3/4-inch – 1 ounce) muffin cups.
Into a medium bowl, sift together the flour, cocoa, baking soda and salt. Mix in the sugar. Make a well in the center. Whisk in the water, oil, vinegar and vanilla extract. Blend until smooth. (The batter will be very thin.) Spoon the batter into the prepared cups. Bake until a wooden pick inserted into the center of one of the cupcakes comes out clean, about 12 to 14 minutes. Cool the cupcakes in the pans on a wire rack for 5 minutes. Remove the cupcakes from the pans and finish cooling on the rack.
Filling: In a heavy medium saucepan over medium-high heat, bring the cream to a boil. Add the white chocolate and remove from heat. let the mixture stand briefly; stir until smooth. Transfer to a metal bowl and refrigerate until chilled thoroughly, stirring occasionally. (To speed the process, set the metal bowl over a larger bowl of ice water; stir the chocolate mixture until cool.) With an electric mixer, beat the white chocolate mixture just until fluffy, about 1 minute.
Transfer the filling to a pastry bag fitted with a 3/8-inch plain tip. Insert the pastry tip 1/4-inch into the bottom of each cupcake and squeeze a little filling into each one.
Glaze: Place the chocolate in a small bowl. Whisk in the boiling water and blend until smooth. One at a time, dip the top of each cupcake into the warm glaze. Turn the glazed cupcakes right side up and set them on a wire rack on top of a baking sheet. Refrigerate the cupcakes for 5 minutes to set the glaze.
Icing: In a medium bowl, whisk the egg white until frothy. Stir in the cream of tartar. Gradually mix in enough of the confectioner’s sugar to make a fairly stiff and smooth icing. Fill a small paper cone with the icing and cut a 1/16-inch opening at the tip. Remove the cupcakes from the refrigerator. Pipe a design (a squiggle, spiral, etc.) on the top of each cupcake. Let the design harden and then cover and refrigerate the cupcakes. Serve at room temperature.
The cupcakes can be made and refrigerated up to 2 days in advance, or frozen up to 2 weeks.
Makes 22 to 24 miniature cupcakes.