Place the icing sugar and ground almonds together in a food processor and pulse for 20 seconds. In a large clean bowl, whisk the egg whites to stiff peaks, then gradually stir in the caster sugar, a tablespoon at a time. Fold in the vanilla essence, followed by a few drops of red food colouring. Next, fold in 1/3 of the ground almond mixture, followed by the remaining 2/3 till you obtain a light, smooth mixture. Pipe the mixture onto a baking sheet covered with non-stick baking paper, slightly apart from each other. Leave to rest for 30 minutes before baking in the oven at 160`c, for 15 minutes only. Meanwhile, prepare the butter cream filling. Mix the butter with the vanilla essence till light. Add in the icing sugar and mix together to a smooth cream. When to macaroons have cooled down completely, sandwich them together with the vanilla butter cream and serve.