Grease an 8 in. (20 cm) round cake tin. Make the pastry in the usual way. Leave to rest while you mix up the filling.
Mix all the dry ingredients for the filling together in a large bowl. Stir in the egg and whisky. Melt the treacle in a pan to make it runny and stir it in.
Roll out two-thirds of the pastry into a circle about 14 in. (35 cm) in diameter. Line the cake tin with it, making sure the pastry comes up above the top of the sides of the tin, Spoon the filling in and level it. Fold the top of the pastry over. Roll out the remaining pastry to an 8 in. (20 cm) circle - you can put the tin on the pastry and cut it round to make an even circle. Moisten the edges of the pastry, put on the circle lid and press the edges firmly together. I use leftover bits of pastry to decorate and to write “Happy New Year” in cut-out letters, which makes the glazed black bun look most impressive.
Lastly, with a skewer, make 4 or 5 holes right down to the bottom of the cake, then prick all over the top with a fork and brush with egg yolk. Bake in the centre of the oven at Gas Mark 4/350°F/180°C for 2½-3 hours. When the pastry turns a rich golden brown lay a piece of foil on top so it doesn’t get any darker. Turn carefully out of the tin and cool. Store in an airtight tin or wrapped in foil in a cool place.