Add the golden syrup, brown sugar and lemon zest into a bowl then set to one side.This is so we can add them into the brown butter straight away, to stop it from continuing to cook. Next, chop the butter into even pieces, add into a medium saucepan then set over a medium heat.
Cook the butter, stirring frequently until it starts to foam, smells nutty and you can see that the milk solids on the bottom of the pan have turned golden brown. This will take 3-4 minutes.
As soon as the brown butter is cooked, take the pan off the heat, immediately add in the golden syrup, brown sugar and lemon zest then whisk to combine. Let this mix cool in the pan until it is barely warm, which will take around 30 minutes.It's key to let the butter & syrup cool before using, otherwise it will make the baking powder react, affecting how the cake rises in the oven! In the meantime, preheat an oven to 170°c/150°c fan (338°f/302°f) and 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!).
When your syrup has cooled, add the eggs, sour cream and oil into a jug then beat with a fork, to combine. Then sift the flour, baking powder & ginger into a large mixing bowl then add in the salt and whisk to combine.
Next, add the syrup mix and sour cream into the flour then whisk by hand, until the batter is smooth & combined. Be careful not to over mix the batter, it should be just mixed, without any lumps.
Once mixed, transfer the batter to the lined tin the pipe a thin line of softened butter straight down the middle. This controls how the cake cracks on top as it bakes.
Bake the cake for 55 minutes - 1 hour, until it is well risen and a skewer inserted into the centre comes out clean.
Once baked, let the cake cool for 10 minutes then poke several holes in the top with a skewer before brushing the 80g of golden syrup on top. If needed, warm the golden syrup slightly to make it easier to brush on.
Let the cake cool/soak in the tin for 20 minutes then run a small knife along each end & carefully lift it out of the tin (using the overhanging parchment). Transfer the cake to a cooling rack, keeping hold of the parchment, as you'll need it later.
Sprinkle a pinch of sea salt on top of the cake, whilst the syrup is still sticky then leave to cool completely.
For the best flavour & texture, it's best to let your cake mature before serving. To do this, wrap the cake in the parchment it was baked in, place another piece on top if needed then wrap tightly in clingfilm. Leave overnight at least before serving but a day or two is best!