Preheat oven to 350. Butter an 8-inch round cake pan, line with parchment paper, butter the paper. Dust paper with a tablespoon of flour and tap out the excess. Cream the butter, sugar and lemon zest for several minutes till smooth and light.
Beat in ricotta and lemon juice. Beat in yolks one at a time until well creamed. Whisk or sift flour, baking powder and salt together in a small bowl. Set aside. Beat whites until they foam, add cream of tartar, then beat the whites till they form stiff peaks. Mix in about one third of the flour into the butter mixture, then fold in half the whites, alternating flour/whites until both are incorporated. Pour batter into prepared pan.
Bake for 30 minutes or until a toothpick comes out clean. Allow the cake to cool for 15 minutes before turning out of pan onto a cooling rack. Peel off the parchment. Turn right side up. Dust cooled cake with powdered sugar. Place a dollop of sweetened whipped cream next to the cake.