Heat oven to 180C/fan 160C/gas 4. Grease and line a deep, 20cm round cake tin.
Beat the sugar and lactose free butter together until light and fluffy, then gradually add the egg, beating after each addition.
Fold in the gluten free flour, cold mashed potato, lemon zest and gluten free baking powder.
Tip into the tin, level the top, then dot the top with frozen raspberries.
Bake for 40-45 mins or until golden and a skewer inserted into the middle of the cake comes out clean.
Turn out onto a wire rack after 10 mins cooling. Mix the granulated sugar and the lemon juice together, then spoon over the top of the cake, letting it drip down the sides. Let the cake cool completely before slicing.