Line two baking sheets with baking paper. Place icing sugar and ground almonds into food processor. Mix to make fine mixture and sift into bowl.
Place egg whites into non-stick bowl and add pinch of salt. Whisk to soft peaks and gradually add sugar until thick and glossy. Divide this mixture into two bowls and add food colouring accordingly.
Fold half the almond and sugar mixture into the macaroon mixture with spatula or metal spoon. Fold in remaining half until thick and shiny. Spoon into piping bag fitted with 1cm-plain nozzle.
Pipe small equal rounds of mixture about 3cm from each other on baking sheets. Put baking sheets gently on work surface and leave for 10-15 mins to form light skin. Bake for 15 minutes and remove from oven to cool.
Beat together butter, icing sugar and flavouring to make buttercream filling. Spoon or pipe on to macaroon and top with other half.
Melt cream and chocolate in bowl over simmering water to make ganache. Stir until combined, then remove from heat and continue stirring until cooled and thickened. Leave to cool before filling macaroon as before.
By Ellen Lavender from The Little Book of Treats by Macmillan Cancer Support (Available exclusively in M&S Cafés for just £3)