Preheat the oven to 180°C / 160°C fan assisted oven / gas mark 4 and line and grease 2 x round 8 inch cake tins.
In a large bowl or food mixer whisk the lemon zest into the sugar until fragrant.
Add the butter one cube at a time and beat into the lemon sugar until pale, light and fluffy.
Add the eggs one at a time, beating on a low speed.
Then mix in the vanilla extract.
Whisk together the flours, baking powder and sea salt in a separate mixing bowl until completely combined.
Add the flour mix alternately with the milk, adding the flour in three additions and the milk in two (begin and end with the flour), scrape down the sides of the bowls as needed and mix until just combined.
Divide the batter between the cake tins and bake for 35-40 minutes when the sponge is coming away from the sides of the tin and an inserted cocktail stick comes out clean.
Leave in the tins for 5 minutes to rest then gently turn out onto cooling racks and leave to cool completely.