Add to the flour mixture, stirring until the mixture just forms dough. Cut as many rounds as possible using a 2-inch fluted biscuit cutter, dipped in flour. Gather the scraps and pat and cut again. Arrange the rounds on the chicken mixture. Beat the egg yolk with the milk and brush the tops of the biscuits with the wash.
Prick biscuits with a fork.
Bake in the middle of a preheated 450f oven for 15-25 minutes, or until biscuits are puffed and golden, and the filling is bubbling.