Preheat oven to 350F/180 C.
Prepare a springform pan by spreading 1 tablespoon (15g) of butter over bottom and sides of pan. Then sprinkle 4 tablespoons of flour and tilt pan so the flour covers all sides. In a large bowl, mix the butter, adding the sugar a little at a time, until the mixture is light and fluffy. Add egg yolks, one at a time while mixing.
Stir in mashed potatoes, cashew nuts, lemon and orange zests, heavy cream and vanilla. Mix well. Add flour gradually while continuing to mix. In another bowl, beat the egg whites until stiff peaks form. Using a spatula, gently fold the egg whites into the batter. Pour the mixture into the cake pan. Bake in preheated over for 1 hour. At the end of baking, poke cake with a toothpick to make sure it is fully baked. The toothpick should come out dry. Wait 10 minutes before unmolding and cutting.