Preheat the oven to 180oC/Gas Mark 4. Grease and line the base of a 20cm (8in) round cake tin.
Combine the ground almonds, potato flour, zest and baking powder.
Beat the butter and sugar in a free-standing mixer until light and fluffy. Add the egg yolks one at a time, beating in between additions.
Tip the almond mixture into the butter and gently mix and fold in.
Whisk the egg whites in a clean bowl until very stiff. Stir 1 tbsp egg whites into the batter to loosen. Fold in the remaining egg whites and pour into the prepared tin. Sprinkle the flaked almonds on top, then bake for 40-45 minutes, until a skewer inserted into the cake comes out clean.
Remove from the tin and sprinkle with icing sugar. Serve with Tims Dairy Rhubarb and Rose Yogurt, if desired.