Grease a 9 inch round cake or springform pan. Line the bottom with waxed paper. Grease paper. Dust with flour. Set aside. Preheat oven t 350°F.
Cream butter and sugar together until smooth. Beat in ricotta.
Beat in egg yolks, one at a time. Add 2 tablespoons of flour and lemon rind and juice. Sift baking powder into remaining flour and beat into the batter until just blended.
Beat the egg whites until they form stiff peaks. fold them carefully into the batter.
transfer the mixture to prepared pan. Bake for 45 minutes, or until tester inserted in center, comed out clean. let cake cool an pan 10 minutes,turn out onto a rack to cool. Dust the cake generously with confectioners sugar before serving.