Arrange an oven rack in the center position and preheat the oven to 180°C. Line a 20 × 20cm baking tray, preferably metal, with foil, pressing it into the corners and smoothing it across the bottom and up the sides. Brush the bottom and sides of the foil with a thin layer of melted butter and set the pan aside.

Place the cashews on a cutting board and chop until the largest pieces are about the size of a pea. Set aside.

Line a small baking tray with a silicone baking mat or baking paper and set it next to the stove. Combine 55 g of the butter, 55 g of the brown sugar, and 30 ml water in a medium saucepan. Set the saucepan over medium heat and bring to a boil, stirring with a heatproof flexible spatula to melt the butter and dissolve the sugar.

Once the mixture is boiling, stir in the chopped cashews and cook, stirring constantly, until the nuts are completely coated and there's no liquid toffee oozing across the bottom of the saucepan, about 5 minutes.

Scrape the cashew mixture out onto the prepared baking tray and spread it into a thin slab with the spatula, then set aside to cool.

Return the saucepan to the stove (no need to wash it). Add the remaining 145 g butter to the reserved saucepan and melt it over low heat. Scrape the melted butter into a large bowl.

Add the remaining 220 g brown sugar and whisk vigorously until the mixture is smooth, thick, and satiny, about 30 seconds.

Add the eggs one at a time, whisking thoroughly after each addition, then continue to whisk until the mixture is completely smooth and slightly pale, about 30 seconds.

Whisk in the vanilla, then add the flour and kosher salt and whisk vigorously until the batter is very smooth and thick, about 30 seconds.

Scrape half of the batter into the prepared pan and smooth it into an even layer with the spatula, working it all the way to the sides and corners.

Break about half of the cooled slab of toffee-covered cashews into small clusters with your fingertips, then scatter the clusters evenly over the surface of the batter.

Dollop the remaining batter on top, then smooth it into an even layer with the spatula.

Break the remaining slab into clusters and scatter across the surface, then sprinkle with flaky salt (if using).

Bake until the surface of the blondies is deep golden brown and firm to the touch, 30 to 35 minutes.

Let the blondies cool completely in the pan, then use the foil to lift them out of the pan and transfer to a cutting board.

Slice with a serrated knife into a 4-by-4 grid to make 16 blondies.