Preheat an oven to 175°c/155°c fan (350°f/310°f).
Grease a 2 lb loaf tin with butter then line the bottom & two longer sides with a piece of baking parchment, leaving a couple of centimetres overhanging the tin's edges (this makes it easier to take the cake out of the tin once it is cooked!).
Sieve the flour, salt & baking powder into a bowl then set aside.
Next, add the caster sugar & lemon zest into a large mixing bowl, rub the zest into the sugar with your fingertips then add in the eggs & yolk.
Using an electric hand whisk, whisk the eggs & sugar for 3-4 minutes, until light & pale then slowly pour in the rapeseed oil, whilst still whisking.
Once the oil has been mixed in, add in half of the sifted flour, half of the double cream & the lemon extract. Fold in with a spatula, until just incorporated.
Add in the remaining flour & cream then fold again, to combine. If there's any lumps, give the batter a gentle whisk, to get rid of them.
Transfer the cake batter into the lined loaf tin the pipe a thin layer of softened butter straight down the middle of the cake.
Bake the cake for 45 minutes - 1 hour, until a skewer inserted into the centre comes out clean. Check the cake after 45 minutes. If it's starting to brown too much, cover the top loosely with foil.