Step 1

Preheat the oven to 160°C. Grease the base and sides of a 23cm x 5cm deep square cake pan and line with baking paper, leaving a 3cm overhang.

Step 2

Place the flour, icing sugar and 1⁄4 tsp salt flakes in a heatproof bowl and stir to combine. Add the melted butter and mix with your hands to form a sticky paste. Press the mixture into prepared pan and smooth out with an offset spatula.

Step 3

Bake for 40-45 minutes until the base is golden. If any bubbles form under the base while baking, use a skewer to gently deflate them.

Step 4

For the caramel filling, place all the ingredients in a large heatproof bowl. Place the bowl on top of a saucepan of gently simmering water, stirring well occasionally, until the butter has fully melted and emulsified. The consistency should be thick and gloopy and the colour just a shade darker – this will take about 10 minutes. Remove and discard the vanilla bean. Set aside at room temperature. As soon as the base is ready, remove from the oven and immediately scrape the caramel filling over the top.

Step 5

Reduce the oven to 120°C. Return the filled base to the oven and bake for 15-20 minutes. If it starts to brown too quickly, reduce the oven slightly and continue to bake. When cooked, the middle will feel set and a little rubbery and the sides should be starting to blister and turn light brown. Remove and leave to cool for 1-2 hours at room temperature.

Step 6

For the topping, melt chocolate and butter in a heatproof bowl over a saucepan of gently simmering water. Stir thoroughly then pour it over the top of the caramel. Smooth lightly with an offset spatula or tilt the pan to distribute the chocolate. Crush the walnuts with your hands and sprinkle over the top with the salt flakes.

Step 7

Choose your ooze factor and cool the slice until the chocolate is set to your liking – glistening and runny (about 15 minutes) or firm (about 45 minutes). Take hold of the excess paper on the sides, then shimmy and lift the slice out of the pan and place onto a chopping board. Peel the paper away from the sides. Dip a serrated knife in hot water, dry and slice into 3cm × 11cm bars. Keeps for 1 day at room temperature, then in the fridge for 1-2 weeks.