ready in 1 hr 20 min.
Mix the flour with the sugar and a pinch of salt and form into a mound on the work surface. Make a well in the centre, break the egg into the well and dot the butter around the edge. Work quickly by hand to form a smooth dough, shape into a ball and wrap in cling film. Chill in the refrigerator for 30 minutes.
Heat the oven to 195°C (175°C in a fan oven), 400°F, gas 6 and line one or two baking trays with grease-proof paper.
Roll the dough out on a floured work surface to a thickness of approx. 0.4 cm and cut into rectangles measuring approx. 4x5 cm using a pastry wheel. Place on the baking tray or trays.
Whisk the milk with the egg yolk and brush thinly over the biscuits. Sprinkle with nib sugar and bake for approx. 10 minutes so that the edges are lightly browned.
Remove from the oven and slide onto a cooling rack with the paper. Once cool, store in an airtight container. Serve with a cup of tea, for example.