Start this recipe by heating the oven to 200`c. Place the caster sugar, ground almonds and lemon zest in a food processor and pulse together for 2 minutes. This process will help to make the almond macaroons lighter. Add in the vanilla essence and 3 egg whites and continue to pulse for 30 seconds. The mixture should be of a soft consistency that holds its shape as it needs to be piped. If the mixture is still too stiff, add in half of the remaining egg white and pulse again. Repeat this process if necessary until you have the correct consistency. Do not be tempted to add in the egg whites all at once because if the mixture becomes too soft or runny the macaroons will flatten out during cooking. Place the mixture into a piping bag fitted with a plain nozzle and pipe the macaroons onto baking sheets covered with rice paper. It is important that the macaroons are piped slightly apart from each other so that they cook evenly. Press a whole blanched almond into the top of each macaroon and bake in the oven for approximately 18-20 minutes. The macaroons should be a light golden brown colour with cracks on top. Remove the macaroons from the oven and allow to cool for 5 minutes before removing the excess rice paper. Allow the macaroons to cool down completely before serving.