Preheat oven to 180ºC . Melt the butter over low heat and set it aside. Mix the eggs and sugar together and place them in a double boiler, stirring constantly by hand until the sugar has dissolved and the mixture is lukewarm. Beat the mixture for about 5 minutes in a mixer on high speed until tripled in volume and pale ivory in color. Sift the flour twice, then gradually sift the flour over the mixture, folding it in gently. Add the cooled butter at the last and fold gently but quickly. Butter and flour the sides and bottoms of two 23 cm springform pans. Pour the mixture into the pans. Bake in a 180ºC oven for about 20 minutes. Remove the cakes from the pans and cool on racks.