Put flour, ground almonds, sugar and salt in a food processor and process for 30 seconds. Add butter and process until mixture resembles fine breadcrumbs. Add egg and lemon essence and process until mixture forms into a ball.
Knead dough on a lightly floured surface until smooth. Pat dough into a disc, put into a plastic bag and refrigerate for 1 hour.
Preheat oven to 160c. Line 2 oven trays with baking paper. Roll out dough between 2 sheets of baking paper until 5 mm thick. Use a reindeer cutter to cut out shapes. Put shapes on prepared trays. Bake for 12 minutes or until cooked through. Leave on trays for 5 minutes, then transfer to a wire rack to cool.
Put royal icing in a small bowl and add enough lemon juice until the icing has the consistency of cream. Add a couple of drops of food colouring and stir to combine.
Put biscuits on a wire rack, spoon a couple of teaspoons of royal icing over each. Spread icing, using a toothpick at edges, until covered. Leave to set for 1 hour. Tie with ribbon, if desired.