1) Bring the cream to a simmer over a low heat in a small saucepan. Combine the sugar and water in another small saucepan over a high heat and cook, without stirring until amber brown.
2) Slowly whisk in the warm cream until smooth and allow to cook for 1 min. Remove from the heat and whisk in the peanut butter, golden syrup, sea salt and 1/8 tsp of vanilla essence until smooth.
3) Transfer the peanut butter caramel mixture to a bowl and allow to cool until thickened at room temperature, about 30 mins.
4) Meanwhile, put a rack in the middle of the oven and preheat the oven to 160C/Gas 3. Line a 23cm by 33cm baking pan with foil, leaving an overhang on the narrow ends. Spray the foil and pan with nonstick spray.
5) Combine the butter, unsweetened chocolate, 60g dark chocolate and espresso powder in a medium bowl set over a pot of simmering water. Let the chocolate melt over a low heat, stirring frequently until smooth. Remove from the heat and allow to cool slightly, about 5 mins.
6) In a large bowl, whisk the eggs, both sugars, vanilla and salt until smooth. Whisk in the melted chocolate mixture until combined. Scrape down the sides of the bowl.
7) Beat in the flour in 2 batches (the batter will be thick). Stir in the remaining 115g of chopped dark chocolate.
8) Scrape half of the batter into the prepared pan and smooth the top. Using a tablespoon, drop dollops of half the peanut butter caramel every 5cm over the top of the batter.
9) Carefully add the remaining batter and smooth it over the top. Put dollops of the remaining peanut butter caramel every 5cm on top of this. Use a butter knife to swirl the caramel through the batter to get a marbleized effect.
10) Bake until the top is set but still soft and the edges are puffed and just beginning to pull away from the sides of the pan, about 23 to 25 mins. A toothpick inserted in the centre will come out still gooey (be brave! underbaking the brownies is one of the secrets to their fudgy texture). Transfer the pan to a wire rack to cool completely.
11) For the neatest cuts, refrigerate the pan for about 20 mins before cutting the brownies. Using the foil, lift the brownie slab out of the pan. Carefully peel off the foil and put the brownie on a large cutting board. With a large sharp knife, cut the brownies into 48 squares.
Note: The brownies can be stored in an airtight container, at room temperature, for up to 3 days; they can also be frozen, well wrapped, for up to 2 weeks.