In a medium pot, stir together the brown sugar, cream and salt over a low heat until the sugar has dissolved completely.
100 ml dark brown sugar, 125 ml (1/2 cup) double cream (heavy cream), 1/4 tsp flaky salt
Turn the heat up to medium-high, bringing the mixture to a boil. Let it simmer while you stir constantly with a spatula, scraping the sides and edges of the pan, for 5-7 minutes, until the mixture has thickened slightly.
Take it off the heat and add the butter and vanilla. Stir until the butter has melted in, then pour the caramel over the baked base and spread it so it covers the entire surface.
45 g (3 tbsp) unsalted butter, 1 tsp vanilla extract
Sprinkle on the chopped walnuts (if using) then chill until the caramel is firm.
a handful walnuts