Preheat the oven to 160°C/325°F/Gas mark 3. Lightly grease an 18 x 28cm/7 x 11in shallow baking tin.
Combine the flour and ground almonds in a bowl. Cut the butter into small pieces, and rub into the flour and almonds with your fingertips until the mixture resembles breadcrumbs.
Stir in the sugar and almond essence, then squeeze the mixture together to form a dough. Put the dough in the prepared baking tin. Spread to the edges by pressing it down with a metal spoon.
Lightly brush the top of the dough with egg white, and sprinkle the flaked almonds over. Bake for 30–35 minutes until lightly browned. Leave to cool in the tin, then cut into individual biscuits. Dust with a little extra sugar just before serving.