- Preheat an oven to 375°F.
- In a heatproof bowl, combine the butter and chocolate. Set the bowl over but not touching barely simmering water in a saucepan. Heat, stirring often, until melted, about 4 minutes. Remove the bowl from the heat and stir in the granulated sugar and salt.
- In a small bowl, whisk together the eggs and vanilla until well blended. Add the egg mixture to the chocolate mixture and stir until well blended.
- Over a sheet of parchment paper, sift together the flour and cocoa powder. Sprinkle the flour mixture over the chocolate mixture and stir until just blended.
- Transfer about one-fourth of the batter to a pastry bag fitted with a wide, round decorating tip. Pipe the batter into the wells of a hearts, X's and O's silicone cake mold (available at "Williams Sonoma" stores), filling each one about three-fourths full. Place the mold directly on the oven rack. If necessary to keep the pan level, place a cooling rack with narrow bars in between the oven rack and the mold. Bake until a toothpick inserted into the center of a cake comes out clean, 12 to 15 minutes.
- Transfer the mold to a wire rack and let cool for 2 to 3 minutes. Invert the mold onto the rack and shake gently to unmold the cakes. Let the cakes continue to cool on the rack.
- Repeat the process three times with the remaining batter. You do not need to wash the mold between each round of baking. Dust the cakes with confectioners' sugar just before serving.