Blend the icing sugar and ground almonds together.
Sift these into a bowl and add 55 grams of egg whites. Stir to combine until you have a sticky mixture. Split the mixture in two and set to one side.
Whisk the remaining egg whites on medium speed, and at the same time heat the caster sugar and water together until mixture reaches 118°C.
At this point, pour the sugar mixture down the side of the stand mixer (the eggs should be foamy) and beat on high speed for 6-7 minutes, until the bowl is cool and the mixture is glossy and has stiff peaks.
Split the meringue in two. Begin folding one half into one of the almond mixtures you made earlier, along with the green food colouring. (You'll need a good squeeze of food coloring to make the macarons nice and bright.) Add the meringue in 2 stages, and fold until the mixture is at the ribbon stage. Repeat with the remaining mixture, this time adding the red food colouring.
Add the mixtures to separate piping bags and pipe into discs onto a silicon mat.
Leave the piped discs to set for 30 minutes until a skin has formed. Bake for 12 minutes at 170°C. Allow to cool completely.
Next, start the buttercream. Beat the butter until it's light and fluffy, then slowly add the icing sugar in 3 stages and continue to beat.
Add the vanilla paste and beat for a further 5 minutes.
Add the buttercream to a piping bag with a round tip nozzle. Pipe a dollop onto one side of each shell. Sandwich with the other shell, and place the finished cookies to one side.
To assemble the tree, pipe a small amount of buttercream on the underside of each macaron. Starting from the base, stick macarons to your foam cone. Alternate with the red and green macarons, wrapping the whole cone until it's completely covered.