In case you want to make this as its own dish (not using leftovers), try this recipe for White Chocolate Mashed Potatoes.
I've never met a potato I didn't like!
MASHED POTATO CHOCOLATE CAKE
4 ounces 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.