Preheat oven to 350°F (175°C).

Grease and flour a 9x13 inch baking pan.

In a mixing bowl, cream butter and sugar until fluffy.

Add eggs, one at a time, beating well after each addition.

Blend in cold mashed potatoes and vanilla extract.

In a separate bowl, combine flour and baking soda.

Add the flour mixture alternately with milk to the wet ingredients, blending well after each addition.

Stir in chopped nuts.

Pour batter into the prepared pan.

Bake for 35-40 minutes, or until a wooden skewer inserted into the center comes out clean.

Let cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Slice and serve.