Preheat the oven to 180C/350F/Gas 5. Grease a 24-26cm/9-10in loose-bottomed cake tin with butter.
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 lemon juice, lemon zest, cinnamon and poppy seeds until well combined.
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.)
Set the cake aside to cool slightly, then serve with crème fraîche.
Type the ingredients you want to use, then click Go. For better results you can use quotation marks around phrases (e.g. "chicken breast"). Alternatively you can search by chef, programme, cuisine, diet, or dish (e.g. Lasagne).