Preheat oven to 400 degrees F. Butter and flour a 9-inch springform pan.
In a large bowl, cream butter and sugar together. Add eggs one at a time; mix well. Add mashed potatoes, 1/2 cup preserves, flour, baking powder, and salt; mix until smooth. Pour batter into prepared pan.
Bake 35 to 40 minutes, or until cake is golden brown and a wooden toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes. Remove sides of pan and cool completely. Serve topped with remaining preserves.