Start by making your caramel. Split the vanilla pod lengthways, scrape out the seeds, then place into a pan.
Combine with the cream, half of the butter and a good pinch of sea salt. Cook, whisking, over a low heat for about 2 to 3 minutes until it’s just bubbling, then take the pan off the heat and set aside.
Place the caster sugar and golden syrup in a medium heavy-based pan over a low heat. Don’t stir, just gently swirl the pan to help dissolve the sugar.
Turn up the heat to medium and keep swirling until the sugar turns into a golden caramel.
Take the pan off the heat. Discard the vanilla pod from the hot milk, then stir in the caramel with a wooden spoon.
Return it to a low heat while you stir in the remaining butter, then remove from the heat and set aside.
Scrunch up a large piece of greaseproof paper, make it wet (shake off the excess), then use it to line a 20cm x 30cm baking tin.
Pour in the caramel, sprinkle with another pinch of salt, and place it on a tea towel in the fridge for 30 minutes, until you have a thick, gooey caramel.
Once the caramel has been chilling for 15 minutes, start the brownies. Preheat the oven to 180ºC/350ºF/gas 4. Grease and line another 20cm x 30cm baking tin.
Melt the butter in a pan over a low heat. Chop and stir in the chocolate until it’s melted, then remove from the heat and stir in the sugar.
Once cooled slightly, whisk in the eggs, then sift and fold in the flour until incorporated. Pour the mixture into the tin.
Take your caramel from the fridge, scoop out spoonfuls and dot them into the brownie mixture, pressing to submerge. Once you have a third of the caramel left, drizzle it on top, using your spoon to ripple it through.
Bake the brownies in the oven for about 25 minutes, or until cooked but still a bit gooey.
Leave to cool for 1 hour, cut into squares and serve.