3 tablespoons butter, softened
3/4 cup sugar
1 egg, at room temperature
1 teaspoon vanilla extract
1 cup lightly packed mashed potatoes (see Notes)
1/4 cup buttermilk
2 cups all-purpose flour, plus more for dusting
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1/4 teaspoon ground cinnamon
3 cups vegetable shortening or oil
Directions1.In a large bowl with an electric mixer, beat butter and sugar until fluffy. Add egg and vanilla and beat until well combined. Add potatoes and buttermilk and beat until smooth. Add 2 cups flour, the baking powder, baking soda, salt, and cinnamon and beat just until evenly mixed. Do not over mix.
2.Generously dust your countertop with flour. Place dough on countertop and turn to coat with flour.
3.Gently press dough out with your hands to 1/2-inch thickness and cut into rounds using a 3-inch donut cutter or biscuit cutter and 1-inch bottle cap for center hole. Gather scraps of dough and gently press out again as needed until all dough is used.
4.In a soup pot, heat shortening over medium heat to 375 degrees, using a thermometer. Working in small batches, cook donuts 2 to 3 minutes per side or until puffed and golden brown on both sides, turning once during cooking. Transfer to a paper towel-lined platter.