Heat the oven to 180C/fan 160C/gas 4. Butter and line the base of a 20cm round cake tin.
Put the egg yolks into a mixing bowl with the caster sugar and, using an electric whisk, beat for 1-2 mins until light and creamy. Stir in the crème fraiche and lemon zest. Add the sifted flour and baking powder (gluten free, if necessary).
In a separate bowl, whisk 4 egg whites until stiff, then fold into the cake mix using the whisk blades.
Pour into the tin and bake for 35-40 mins until the cake feels firm to the touch. Cool in the tin for 5 mins, then turn out and cool completely on a wire rack. Dust with icing sugar and serve.