1.Beat egg whites in small bowl with electric mixer until soft peaks form. Add caster sugar; beat until sugar dissolves; transfer mixture to large bowl. Fold in sifted icing sugar, ground almonds and sifted cocoa, in two batches.
2.Spoon mixture into piping bag fitted with 1cm plain tube. Pipe 4cm rounds about 2.5cm apart onto trays. Tap trays on bench so macaroons spread slightly. Stand 30 minutes.
3.Preheat oven to 150°C/300°F. Bake macaroons about 20 minutes. Cool on trays.
4.To make the dark chocolate ganache: Bring cream to the boil in small saucepan. Remove from heat; pour over chocolate in small bowl, stir until smooth. Stir in nuts. Stand at room temperature until spreadable.
5.Sandwich macaroons with dark chocolate ganache.
Buttered Brazil nuts are toffee-coated nuts. They’re available from nut shops and gourmet food stores.