For the Filling: In medium bowl combine melted butter (slightly cooled), sugar, egg yolks, and lemon zest. Whisk until smooth. Pour into a double-boiler and set over pan of simmering water. Stirring every few minutes, cook 30-45 minutes, until mixture thickens and is smooth. Remove from heat, let cool slightly, and pour into heat-resistant bowl. Cover with plastic wrap and let sit in fridge 2 hours minimum.

For the Cake: While the curd is cooking, butter and flour 2 9” cake pans and set aside. Preheat the oven to 325 F. In medium bowl whisk together flour, baking powder, and salt. In large bowl beat egg yolk using an electric hand-mixer until frothy, about 2 minutes. Add in sugar in three stages, beating until eggs become pale and smooth. Stir in lemon juice and zest.

In a separate large bowl beat the egg whites and cream of tartar on high until egg whites are stiff.

Pour half of the flour mixture into the egg yolk mixture and fold, then pour in half the egg white mixture and fold again very gently. Repeat with remaining flour-egg white mixtures until all ingredients are combined. Pour batter evenly into prepared cake pans and bake 22-30 minutes, until toothpick inserted in cake comes out clean. Remove cakes from oven and let sit five minutes to cool, then release cakes from pans and set on wire rack to cool completely.

For the Icing: In large bowl beat the butter 2-3 minutes using an electric mixer. Pour in juice, zest, and 2 c. Sugar and beat on medium-high until combined. Continue adding sugar until the icing is thick and spreadable. For decoration slice the cakes in half horizontally, creating 4 layers. Spread the filling thickly over the middle two layers and cover cake with thin coat of frosting (this is called the crumb coat). Place the cake in freezer for an hour until crumb coat is set, then remove and finish icing cake. You can cut some thin slices of lemons and oranges for decorating the top.