Preheat oven to 170°C conventional / 150°C fan force and line a 20 cm square baking pan with baking paper.
Place the 150 g butter in a saucepan over medium heat. Cook until it foams and the milk solids turn deep golden brown and smell nutty.
Immediately pour into a bowl, including all the browned milk solids. Cool for 10–15 minutes.
Process the roasted pistachios until finely ground. Don’t take them so far that they become a paste.
Combine the ground pistachios, flour, brown sugar, caster sugar and salt.
Add the browned butter and vanilla and stir until you have a soft, crumbly dough.
Press the dough firmly and evenly into the lined pan. Bake for 20–23 minutes, until golden around the edges and lightly coloured across the surface. Cool completely.