Wash and dry the lemons; grate the peel without going into the white pith, and then squeeze them to obtain the juice. Put the ricotta through a sieve to prevent lumps forming and then, in a bowl, combine it with the sugar and salt, mixing carefully with a whisk (or an electric hand mixer if you prefer). Add the eggs, one at a time, beating continuously until the mixture becomes smooth and soft. Sift the flour, starch and baking powder together, and combine them with the rest of the mixture. Add the butter (previously melted in a “bain-marie”) and, last of all, the grated peel and lemon juice; continue stirring until the mixture is smooth. Pour it into a round, springform cake tin, lined with greaseproof paper, buttered and dusted with flour (or even directly into the tin), and gently jiggle the tin to level the mixture. Bake in a preheated oven, at 180 °C, for 40 minutes; if the cake begins to brown too much, half way through cooking, cover with baking foil. Turn the oven off and leave the cake in it for another 5 minutes; a nice golden crust should have formed on the surface. Remove from the tin when lukewarm, leave to cool down and then sprinkle with icing sugar before serving (it should remain moist inside). You can serve the cake with strawberry, blackberry or blueberry jam, warmed slightly with 1 or 2 tablespoons of water.