Preheat the oven to 350 degrees F. Beat egg yolks well, about 4 to 6 minutes. Add sugar and continue beating well, another 4 to 6 minutes. Sift together the flour and the baking powder. Gradually add dry ingredients and lemon juice and rind to wet mix. Beat egg whites to medium peaks and fold into batter. Butter and flour 3 (9-inch) cake rounds. Place a parchment paper circle in the bottom of each cake round. Pour the batter into the prepared cake rounds and bake at 350 degrees for 30 minutes. Cool cakes completely. With a serrated knife, slice each cake into 3 layers.
Cream all ingredients until smooth. Frost each layer and the outside of cake. Cover the whole cake with crushed walnuts. Refrigerate cake for 2 hours before slicing.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.