Preheat oven to 350°F.
In the bowl of a stand mixer or in a large mixing bowl, cream together butter and sugar until pale and fluffy.
Add eggs, one at a time, until fully incorporated. Beat in mashed potato and vanilla.
In a separate, smaller bowl, combine the dry ingredients – flour, cocoa powder, and baking soda.
Alternating, add half of the dry ingredients and then half of the milk, beating to combine between additions and repeating until all the ingredients are incorporated.
Spread batter evenly between two 9” round cake pans (or one 9″x13″ rectangular pan) that have been greased and lined with parchment paper.
Bake at 350° until a toothpick inserted in the center comes out clean, about 25-30 minutes.
Cool pans on a cooling rack for 10 minutes before removing the cake to wire racks to cool completely (about 30 minutes).
Serve topped with whipped cream, a dusting of powdered sugar, or fresh fruit, or stack in layers and ice with icing of choice.