Cream the butter and sugar together, add the lemon zest. Whisk in the egg yolks one by one and the flour sifted with bicarbonate of soda and baking powder. Preheat the oven to 300F. Beat the egg whites until firm with the salt and carefully fold into the mixture. Butter and flour a 9-inch cake pan and pour in the cake mixture. Bake for 20 minutes without opening the door. The cake will rise up and fill the pan. If it gets too brown cover with paper. Insert a knife and if it comes out clean the cake is cooked. The directions for this four by four cake produces a soft texture.
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4 eggs and their weight in sugar, flour, unsalted butter
