Put the egg whites in a clean, medium-sized bowl and leave at room temperature for 10-15 minutes.
Meanwhile, put the icing sugar and ground almonds together in a blender and whizz together for 1-2 minutes until a fine powder. Sieve this powder onto a plate, discarding any rough pieces of almond that may remain in the sieve.
Heat the oven to 150C, fan oven 130C, gas mark 2. Using an electric whisk, beat the egg whites into very soft peaks. Slowly add the caster sugar, whisking all the time until the egg whites become very stiff and shiny.
With a spatula or large metal spoon, fold about a third of the almond mixture into the meringue, with 3-4 drops of the gel colouring. Fold until just mixed in.
Fold half the remaining almond mixture into the bowl until just mixed. Now, fold the remaining almond mixture in and keep folding and stirring until the mixture looks like loose wet sand and you can make a figure of eight with the mixture with the spatula or spoon.
Line two baking sheets with baking parchment and either pipe out small even rounds of mixture or drop even-sized amounts from the tip of a teaspoon.
Bang the baking sheets on the work surface to get rid of any air bubbles, then leave them at room temperature for an hour. This allows the macarons to form a skin on top before baking.
Bake for about 10-15 minutes, depending on size. They should be firm enough to lift easily off the baking sheets. Cool on a wire rack.
For the buttercream beat together all the ingredients until smooth making your own colours and flavourings. Chill for 30 minutes before spreading a little on the base of half the macaron shells. Sandwich together with the remaining macarons and smooth off the filling. Chill for about 1 hour before serving.
Blending and sieving the almonds and icing sugar helps create the traditional smooth macaron tops.
Make the mixture slightly stronger in colour than you want it, as it fades on cooking.
Any left-over buttercream will freeze for your next batch of macarons or cakes.
The macarons should be left to chill after filling to allow the centre to soften and become chewy. They freeze well for up to 2 months. Allow 30 minutes at room temperature to thaw.
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