Preheat oven to 180*C. Lightly grease 20x30cm baking tin and line with baking paper.
Place the butter, sugar and flour in a large bowl and rub together with the tips of your fingers, until the mixture starts to resemble moist rough breadcrumbs.
Press the mixture firmly into the base of the prepared tin and bake for 15 mins or until a light golden colour. Set aside. Once cool, spread with the dulce de leche. Stir the salt in with the pecans then scatter these over the top. Chill in the fridge for 30 minutes, or until set.
Melt the chocolate and butter in a bowl set over a pan of simmering water. Stir well and pour into the tin, smoothing it out with a palette knife to make an even top layer of chocolate. Chill for at least 30 minutes, then cut into squares and serve.