Line a 22 cm square tin with baking paper, lightly grease with butter or oil spray.
Place all base ingredients into a food processor. Pulse 5 or 6 times, then process until the mixture comes together. Pour the mixture straight into the lined baking tin and press evenly into the bottom.
Bake for 25 minutes as you make the topping.
Place topping ingredients, except the nuts, into a small saucepan over low-medium heat. Stir occasionally as it melts and simmers and all the icing sugar has dissolved.
Pour the topping, while still hot, over the base. The base should also still be a little warm. Set aside for 5 minutes and then sprinkle over the chopped pistachios.
While the slice is still warm, cut into portions and keep them in the tin as one slab. This makes cutting easier as it can be quite hard once completely chilled. Also, cutting it chilled runs the risk of it shattering into small pieces.
Cover with plastic wrap and refrigerate overnight. To serve, slice through the previous cuts and serve cold or at room temperature.