Beat the butter and brown sugar until creamy. Add the egg yolk, ground mixed spice and flour and mix to form a soft dough. Lightly beat the egg white to break it up and tip it on to a plate. Scatter the almonds on a separate plate.
Shape the dough into small balls, 3 cm (1¼ inches) in diameter, and roll them first in the egg white and then in the almonds until well coated.
Place the balls on a greased baking sheet, spaced slightly apart, and flatten slightly. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10 minutes, then remove from the oven. Allow to cool a little, then lightly flour your thumb and make a thumbprint in the centre of each cookie. Spoon a little jam into each cavity and return the cookies to the oven for an extra 10 minutes or until pale golden. Transfer to a wire rack to cool.
Dust the edges of the cookies with icing sugar, if liked. They will keep, in an airtight container in a cool place, for a few days.