Melt butter and 8 ounces chocolate in a heavy saucepan over medium-low heat, stirring often. Remove from heat, and cool completely. Stir in sugar, whipping cream, and rum extract until blended. Add eggs, 1 at a time, stirring until blended after each addition. Gradually fold in flour. Stir in mini-morsels.
Spoon batter into lightly greased miniature (1 3/4") muffin pans, filling almost full. Bake at 375° for 14 minutes or until a wooden pick inserted in center of cakes comes out clean. Remove to a wire rack to cool. Sprinkle cakes with powdered sugar before serving.