Place the fats and sugar in a large bowl and beat together, then beat in the egg yolk followed by the remaining ingredients. Form the dough into a ball, wrap closely in clingfilm, and chill for 1 hour.
Remove the dough from the refrigerator, unwrap and place on a surface dusted lightly with flour. Knead the dough a little to soften it, then divide into 20 balls.
Place the balls on two baking sheets, flatten them slightly with a fork and place in a preheated oven, 180°C (350°F), Gas Mark 4, for about 20 minutes until golden. Remove the biscuits from the oven and transfer to a wire rack to cool. Dust over a little icing sugar before serving.