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.