Preheat the oven to 180C or 160C fan.
Butter a 20 cm baking round cake tin and line it with a baking paper. In the end, butter on top of the paper as well to make sure that the cake won't stick.
Using a hand mixer, beat the sweetener and Lurpak spread until fluffy. Then add the effs, one by one, beating after each addition.
Mix in the flour, cold mashed potato (make it fluffy with a fork first), lemon zest and a baking powder.
Place the batter in the tin, levelling it. Bake in the oven for 40-45 minutes or until a skewer comes out clean (it depends on your oven).
Cool it for 10 minutes, then turn it upside down. In a bowl, mix the sweetener with the lemon juice until dissolved. Using a fork, make lots of little holes in the top of the cake. Drizzle the sweet and zesty juice over the cake. Serve!