Preheat the oven to 180 C / 160 C fan. Grease and line a 23 cm springform tin and set aside.
Cream the butter and sugar together in a bowl until pale and fluffy. Whisk in the mashed potato and ricotta using an electric whisk for 4-5 minutes, or until very fluffy. Crack in the eggs, one at a time, and whisk again until fully incorporated into the mixture. Sift the flour and baking powder together, then fold the mixture into the cake batter until well combined.
Fold in the juice of 1 lemon and the zest of 2 until well combined then spoon the cake batter into the prepared cake tin.
Bake for 1-1¼ hours, checking regularly, until a skewer inserted into the centre of the cake comes out clean. (If the top of the cake starts to darken too much during cooking, cover it loosely with aluminium foil.)
Mix together the juice of the remaining lemon and the icing sugar then pour over the hot cake still in it’s tin. Set the cake aside to cool slightly, then serve in slices.