Preheat oven to 130C. Grease oven trays lined with baking paper.
Beat egg whites in a small bowl with an electric mixer until soft peaks form.
Add caster sugar, beat until sugar dissolves, transfer mixture to a large bowl.
Fold in sifted icing sugar, ground almonds and sifted cocoa, in two batches.
Pour mixture into a piping bag fitted with a 1cm plain tube. Pipe 4cm rounds about 3cm apart onto trays. Tap trays so macaroons spread slightly. Stand for 30 minutes.
Bake macaroons for about 20 minutes. Cool on trays.
Dark Chocolate Ganache
Bring cream to boil in a small saucepan. Remove from heat, pour over chocolate and stir until smooth. Stand at room temperature until spreadable.
Sandwich macarons with the chocolate ganache.