For the cake, dust a 9 inch spring form pan with flour. Preheat oven to 325 degrees - same for a convection oven. Sift flour, baking powder, and salt together. Beat egg yolks until thick and light (about 5 to 7 minutes). Blend sugar into egg yolks gradually until fully combined. Blend in lemon juice to egg yolks. Beat egg whites until just stiff. Gently fold half of the egg whites into the egg yolks until just incorporated. Fold in half of flour mixture into egg yolks. Repeat with remaining egg whites, then remaining flour mixture. Fold gently; Do not beat, whip, or stir harshly. Pour batter into cake pan and bake for 45 minutes or until cake is firm and springs back when touched. Place cake pan on rack and allow to cool to room temp, about 2 hours. Once completely cooled, slice into two sections of equal thickness. *Reserve crumbs from cake pan and brush some from the cut slices of cake for topping. Return bottom section to the spring form pan and re-secure rim (this will aid in filling the cake).
Lemon Cream filling: While cake is baking, combine the egg yolks, sugar, flour, milk, and lemon oil in the top pan of a double boiler- use an electric mixer and beat until smooth, about 6 minutes. Place pan on top of base pan with boiling water, and whisk for 5 minutes, or until smooth and thick. Remove from heat and allow to cool for 10 minutes. Transfer to the fridge for about 15 minutes to cool completely. Once cooled, fold onto cake bottom in springform pan; Stack cake top on cream filling. Combine reserved crumbs with equal amount of confectioners sugar, and dust top of cake. Return cake to fridge for at least 2 hours to allow cream to set, better if overnight.