Warm the milk in a saucepan with the saffron. Once steaming, take off the heat and whisk in the melted butter, then put in the fridge to chill completely. Whisk the egg with the caster sugar until light and fluffy – this will take around 3-5 mins, then add the cooled butter and light brown sugar and whisk briefly to combine. Tip in the flour, baking powder and a big pinch of salt, and use a spatula or the paddle attachment of a stand mixer to combine – be careful not to over mix it.
Fold in the chocolate and pistachios, then tip the mixture onto a piece of baking parchment. Roll it up in the parchment into a thick, compact log and twist the ends to make a cracker shape. Wrap in cling film, then put in the fridge for at least a couple of hours to chill – leave it overnight if you have time.
When ready to bake, heat oven to 190C/170C fan/gas 5 and line a baking sheet with baking parchment. Unwrap the cookie log and use a sharp knife to slice into thick 1cm disks, space out on the sheet (you might need to do this in two batches) and bake for 12-15 mins until set and golden around the edges, but still soft. Allow to cool a little, then move to a wire rack to cool completely. Will keep for 2-3 days in an airtight container.