Sift together the ground almonds and icing sugar to remove any clumps. Blend any leftover mixture in a food processor then sift again until nothing remains.
Begin beating the egg whites and cream of tartar on low speed. Once the egg whites are very foamy, start sprinkling in the sugar as you beat. Increase the speed to medium, if necessary, and beat the meringue to stiff glossy peaks.
Add about 1/4 of the almond/sugar mixture and fold until no streaks remain. Continue to add the almond mixture in quarters, folding until incorporated. Tint the batter bright red. The batter should be the consistency of molten lava.
Pour the batter into a piping bag fitted with a straight nozzle and pipe rows of batter about 3cm wide onto the baking sheets, giving them space to spread.
Drop the baking sheet firmly on the counter from a height of about 20cm firmly, repeat 3 times. Allow the cookies to rest on a level surface at room temperature for 30-60 minutes until they are no longer tacky to a light touch.
While they rest, place an oven rack in the lower 3rd of your oven and preheat to 150 °C (fan-forced). Bake the cookies for 16-20 minutes. If the tops are firm and no longer move away from the bottoms, then they are ready.
To make the buttercream, add two cups of coke to a large pot and boil it down for 20-30 minutes stirring infrequently. After 20-30 minutes the coke should be reduced to a runny syrup consistency, chill in the refrigerator for about 15-30 minutes until cool to the touch.
With an electric mixer, beat butter on medium for 1 minute. Add the coke and mix thoroughly. Add the icing sugar, milk and food colouring. Sandwich the macarons together using the buttercream.
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