In case you want to make this as its own dish (not using leftovers), try the recipe I posted for White Chocolate Mashed Potatoes.
I've never met a potato I didn't like!
MASHED POTATO CHOCOLATE CAKE
4 oz. dark chocolate (65-75% cacao)
1 cup sweet butter
1 3/4 cups sugar
1 cup mashed potatoes
1 cup buttermilk
1 tsp. Madagascar or Mexican vanilla extract
2 cups cake flour
1 tsp. baking powder
1/2 tsp. baking soda
1/2 tsp. salt
2 ounces dark chocolate chopped into chunks (or chocolate chips)
Melt chocolate with vanilla; cool slightly.
Sift together flour, baking powder, soda, and salt.
In another bowl, cream together butter and sugar and beat in eggs, one by one.
Add chocolate and mashed potatoes.
Beat in dry ingredients alternately with buttermilk.
Fold in dark chocolate chips or chunks.
Pour into greased 13x9 x 2 inch pan (or Bundt Pan) and bake in preheated 350 degree oven for approximately 40 minutes.