Modern and sophisticated, these miniature chocolate-swirl wedding cakes can be made in a myriad of ways and are both a decorative and delicious addition to your wedding reception.
Preheat oven to 350 degrees. Butter 16 4-inch-diameter, 2-inch high cake pans (2 for each cake). Line bottoms with parchment paper; butter parchment, and dust with cocoa, tapping out excess. Stir cocoa powder and the boiling water in a small bowl until a smooth paste forms; let cool. Sift together flour, baking soda, and salt into a medium bowl.
Cream butter and sugar in a mixer bowl until pale and fluffy, 3 to 4 minutes. Mix in vanilla. Add eggs in a slow stream, mixing until batter is no longer slick.
Gradually whisk milk into cocoa paste. With mixer on low, gradually add flour and cocoa mixtures. Pour into pans. Bake, rotating halfway through, about 30 minutes, until a cake tester inserted into centers comes out clean. Let cool on racks 5 to 10 minutes before turning out onto racks; let cool completely.
Mix buttercream and bittersweet chocolate. For each cake, trim top level. Spread 1/4 cup buttercream on top of one layer, and set second layer on top. Spread 1/4 cup buttercream over entire cake, then refrigerate until firm, about 30 minutes.
Pour white chocolate into a cornet or pastry bag fitted with a very thin tip (such as Ateco #1).
To make the glaze, place chocolate in a medium heatproof bowl. Stir cream and corn syrup in a medium saucepan over medium heat, until just beginning to boil. Pour over chocolate; let stand a few minutes, then stir until smooth. Let cool slightly; use immediately to pour over cakes. Excess glaze can be returned to pan. If glaze becomes too thick to pour, set bowl over a pan of simmering water to reheat, stirring constantly.
Place cakes on a wire rack with parchment paper below. Working with one cake at a time, pour about 3/4 cup glaze on top and around sides of cake (tap rack to help chocolate fall to bottom). Immediately pipe white chocolate on top of cake. For stripes, pipe lines of white chocolate, and drag the tip of a skewer or toothpick across them (alternating directions creates zigzags; drawing them all the same direction creates scallops). Wipe tip of skewer before each new line. Or pipe circles, then draw lines between center and edge, alternating directions. Scrape excess glaze from parchment to use on remaining cakes, reheating if necessary. Let cakes sit at room temperature up to 2 hours before serving.