Grease a round cake pan and line the bottom with baking paper. Grease the paper and dust with flour. Preheat the oven to 180ºC. Soften butter and sugar together, add ricotta and beat. Add the egg yolks one at a time and keep beating. Add 2 tbsp of flour, the lemon zest and the juice. Mix the baking powder into the remaining flour and add to the mixture. Beat the egg whites until they are stiff and fold them into the batter. Pour the mixture into the pan and bake for 45 minutes. Let the cake cool for 15 minutes, turn it out onto a rack and dust with the icing sugar. Serve and enjoy!