Heat oven to 350 degrees. Grease a 10x15-inch baking pan and dust it with flour. In a large bowl, using an electric mixer, beat butter with both sugars until light and fluffy. Beat in eggs. Add mashed potatoes and vanilla; continue to beat until smooth. In a mixing bowl, sift or whisk together the flour, cocoa, salt, and baking soda. With the mixer on medium speed, add the flour mixture alternately with the buttermilk, beating just until smooth. Stir in the chocolate chips and nuts. Scrape batter into prepared pan, smoothing the top. Bake in preheated oven 22 to 25 minutes, until the top is springy when lightly touched and a tester inserted in the center comes out clean. Cool in the pan on a wire rack for about 20 minutes. While still warm, cut into 24 or 36 squares. Dust with powdered sugar before serving. These squares can be stored covered at room temperature for two days, or can be frozen.
*For this recipe you can use instant mashed potatoes, prepared according to package directions. Or, if you have them on hand, use leftover mashed potatoes. You can also use regular milk.