Preheat the oven to 155'C.
Sift the ground almonds and the icing sugar together.
Whisk the egg whites until white and fluffy and then add the caster sugar to the egg whites and continue whisking until you have stiff, glossy peaks.
Fold the ground almonds and icing sugar into the meringue mixture.
Cut and fold the mixture with a spatula until all of the dry ingredients are incorporated and the mixture is smooth, shiny and has reached the ribbon stage.
Carefully add the food colouring making sure not to overwork the mixture. Spoon the mixture into a piping bag.
Pipe 2cm circles in straight lines across a baking sheet lined with a silicon paper. Leave a 1 cm gap between each macaroon shell. Drop the tray of macaroons onto the work surface from about 30 cm to remove any large air bubbles.
Allow the macaroons to rest at room temperature for 20 minutes or until the macaroon shells are no longer sticky to touch.
Bake in the oven for 15 minutes. If you are baking lots of macaroons you may need to keep the oven door slightly ajar (with a spoon or paper) to prevent steam building up in the oven. After 15 minutes, check the macaroons are cooked by gently lifting a macaroon at the edge of a tray. If the test macaroon does not stick, allow the tray of macaroons to finish cooking on the tray as it cools.
Allow the macaroons to cool completely before filling.
Wash the rhubarb and then cut it into as small dice as possible.
Place the egg yolks and the cornflour in a large mixing bowl.
Heat a large saucepan and add the rhubarb pieces and the caster sugar. Stir the mixture continually and, when it starts to break down and resemble a puree, add the milk and the double cream. Keep stirring the mixture until it comes to the boil. Slowly pour the mixture on to the egg yolks and the cornflour. Combine well and then return to the pan and begin to whisk. Place the pan back onto the heat and bring to the boil. Once boiling, keep on the heat for a further minute and then transfer to a bowl and chill in the fridge.
Once the mixture is completely cool, transfer to a food processor and blend until smooth and shiny. Transfer to a piping bag and then pipe on to the flat side of half of the macaroon shells. Top with a second macaroon shell and twist gently.
«Sometimes when you cook rhubarb it can turn slightly brown in colour, if this happens, add some red food colouring to the custard mixture to make sure it is a nice attractive colour.»