1) Whisk the dry ingredients except the sugar. In the bowl of an electric mixer fitted with a paddle, cream the butter with the sugar until slightly fluffy. Slowly beat in the milk.
2) On low speed, beat in the dry mixture until combined; dough will be sticky. Divide the dough into 4 equal portions, roll each to 1/2-cm thick between sheets of greaseproof paper, and freeze until firm, about 20 minutes.
3) Preheat the oven to 160C/Gas 3. Line sheet trays with greaseproof paper.
4) Put the dough on a floured work surface; cut and transfer biscuits to the prepared sheets. Reroll and cut the scraps into biscuits.
5) Bake the thinner biscuits for about 11 minutes, until crisp, and the thicker about 20 minutes. Cool for about 2 minutes, then transfer to a rack to finish cooling.