3 1/2 ounces semisweet chocolate, melted and cooled
Two 9-inch round baked chocolate cake layers, warmed
4 ounces white chocolate, chopped
7 ounces extra-bittersweet chocolate, chopped
1/2 cup toasted pumpkin seeds
About 1 dozen black licorice whips, cut into 3-inch lengths
In a large bowl, beat the cream cheese with the butter, 1/2 cup of the confectioners' sugar and the melted chocolate. Crumble 1 1/2 of the warm cake layers into the bowl and beat at low speed until incorporated. Roll level tablespoons of the mixture into 3 dozen ovals, tapering one end to shape the nose. Arrange the mice on a wax paper–lined baking sheet; press a toothpick into the tail end of each. Refrigerate until firm, 30 minutes.
Meanwhile, preheat the oven to 275°. Crumble the remaining 1/2 cake layer and spread on a rimmed baking sheet. Bake for 30 minutes, stirring frequently, until almost dry. Very finely crush the crumbs.
In a microwave-safe bowl, heat the white and extra-bittersweet chocolate together on high power until three-quarters melted. Stir the chocolates until completely melted and cooled to 90° on an instant-read thermometer. Working quickly, and using the toothpick as a handle, dip the mice into the chocolate to coat; let any excess drip back into the bowl. Roll the mice in the cake crumbs and press 2 pumpkin seeds into the heads of each mouse to make ears. Refrigerate until firm, about 20 minutes.
In a bowl, combine the remaining 1/2 cup of sugar with water to make a stiff glaze. Transfer to a small plastic bag and snip off a tiny corner from one end. Pipe eyes on the mouse. Remove the toothpicks and insert a licorice whip into each indentation for the tails.
The chocolate mice can be refrigerated for 4 days.