Put the flour, ground almonds, sugar and almond essence in a mixing bowl or a food processor. Add the butter and rub in with your fingertips or process until the mixture resembles fine breadcrumbs.
Squeeze the mixture together with your hands to form a ball. Knead lightly then roll out on a lightly floured surface until 1 cm (½ inch) thick. Stamp out 6 cm (2½ inch) circles using a fluted round biscuit cutter. Transfer to an ungreased baking sheet. Reknead the trimmings and continue rolling and stamping out until all the mixture has been used.
Prick each shortbread biscuit 4 times with a fork to make a cross shape then add an almond half to the space between each fork mark. Decorate the centre with a small piece of glacé cherry. Sprinkle with a little extra caster sugar and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for about 15 minutes until pale golden.
Loosen the biscuits and leave to cool on the baking sheet or transfer to a wire rack if preferred.