Preheat the oven to 200°C/180˚C fan/gas mark 6. Grease a 20cm square loose-bottomed cake tin and line with baking paper.
First make the topping. Put the hazelnuts into a small dry frying pan and set it over a high heat. As the pan warms up, toss the hazelnuts every so often, until they start to brown and their ghostly smell begins to haunt. Tip into a bowl to cool, then roughly chop them.
Wipe out the frying pan and set over a medium-high heat. Sprinkle the sugar into the base of the pan and allow it to melt and turn brown - if the sugar starts to brown more quickly in some areas, just give it a gentle stir. Once you have a fairly dark-amber caramel, throw in the butter and stir it vigorously, then add the cream, a drop at a time, stirring. If the caramel solidifies, you’ve added the cream too quickly, in which case just add all of the cream, reduce the heat to medium and stir for a while until the sugar melts into the cream.
Once you have a smooth sauce, bring it to a boil for just a minute, then turn off the heat and add a pinch of salt flakes. Pour into a cold bowl and set aside until needed.
For the brownie batter, put the butter into a saucepan and set over a high heat. Allow the butter to sizzle and melt, swirling the pan every so often as it bubbles and spatters. Once the bubbles turn into a fine, cappuccino-like foam and the butter smells nutty, remove it from the heat.
Set a heatproof bowl over a pan of barely simmering water and add the chocolate to the bowl. Allow the chocolate to melt, then remove the bowl from the pan and add the browned butter, sugar and flour and beat until smooth. Add the eggs and beat until the mixture is extremely smooth and velvety.
Pour the mixture into the prepared cake tin and drizzle over 3 tablespoons of the salted caramel sauce. Scatter over half of the chopped hazelnuts. Bake for 30-35 minutes, until the brownie is puffed and set.
Unlike a properly baked cake, this will still be fairly fudgy inside, so the skewer test is redundant; instead I eat what comes out on the skewer and taste for starch. If the batter still tastes a little floury, bake it for a little longer until it just tastes chocolaty and rich. When the brownie is baked, allow it to cool in the tin for 15 minutes, then remove the brownie still on the cake tin base, from the tin. Put into the fridge overnight, or the freezer for an hour or so. Once the brownie is firm, drizzle over more sauce, and scatter over the remaining hazelnuts. Cut into 16 equal pieces - a long chef’s knife dipped into a tall jug of very hot water helps to slice the brownie neatly - just wipe the blade of the knife after each cut, and re-dip into the water. Serve immediately or store in an airtight tin for up to a week.