Heat oven to 160C/140C fan/gas 3. Line a muffin tin with 12 muffin cases. With an electric whisk, whisk the butter, 250g of the sugar, eggs, flour, zest of 2 lemons and juice from 1 lemon until just combined - don’t over-whisk. Divide between the cases (they will be quite full), then bake on a middle shelf for 30 mins, until a skewer poked in comes out clean.
Cool for 10 mins, then transfer cakes to a cooling rack on a tray and poke each a few times with a skewer. Pour remaining lemon juice over remaining 150g sugar and zest 1 lemon, and immediately spoon over the cakes. Leave to cool.