Preheat the oven to Gas 3, 170°C, fan 150°C. Butter and flour a 30 x 11cm (12 x 4in) loaf tin.
Whip the butter and sugar together with electric beaters until creamy. Add the vanilla and eggs one at a time, beating well after each one. The mixture may look lumpy for now but will come together fine later.
Whisk in the lemon zest, then the flour and baking powder. Whisk in the lemon juice and milk – it should thin out to a thick batter.
Spoon big dollops of mixture along the length of your tin. You don't need to smooth it out, it will do that as it cooks. Bake for 50-55 minutes or until the cake is golden and crusty and a skewer poked into the middle comes out clean. Cover with foil for the last 15 minutes if it is browning too much.
Loosen the sides with a knife and turn out the cake, then put it back in the tin to cool completely (just to make sure you won't have problems getting it out later). This is a good dense cake that will keep well for several days.
Recipe from Piri Piri Starfish by Tessa Kiros, published by Murdoch Publishing, £22·50. Click here to buy