Grease a 9 inch round cake or springform pan. Line the bottom with parchment or waxed paper. Grease the paper. Dust with flour. Set aside. Preheat the oven to 350 degrees.
Cream the butter and sugar together until smooth. Beat in the ricotta.
Beat in the egg yolks, one at a time. Ass 2 Tbl of the flour, and the lemon zest and juice. Sift the baking powder into the ramaining flour and beat into the batter until well blended only.
Beat the egg whites until they form stiff peaks. Fold them carefully into the batter.
Turn the mixture into the prepared cake pan. Bake for 45 minutes, or until a cake tester inserted in the center of the cake comes out clean. Allow the cake to cool for 10 minutes before turning it out onto a rack to cool. Dust the cake generously with confectioner's sugar before serving.