1.Grease oven trays; line with baking paper.
2.Blend or process icing sugar and ground hazelnuts until fine. Sift mixture; discard any coarse pieces in sifter.
3.Combine coffee and the boiling water in small bowl.
4.Beat egg whites in small bowl with electric mixer until soft peaks form. Add caster sugar slowly, beating until sugar dissolves; transfer mixture to large bowl. Fold in sifted icing sugar, ground hazelnuts and coffee mixture, in two batches.
5.Spoon mixture into piping bag fitted with 1cm (½-inch) plain tube. Pipe 4cm (1½-inch) rounds, about 2.5cm (1 inch) apart, on trays. Tap trays on bench so macarons spread slightly; stand about 30 minutes or until macaroons feel dry to touch.
6.Preheat oven to 150°C/300°F.
7.Bake macarons about 20 minutes. Cool on trays.
8.Meanwhile, for white chocolate ganache, bring cream to the boil in small saucepan. Remove from heat; add chocolate, stir until smooth. Stand at room temperature until spreadable.
9.Sandwich macarons with ganache. Serve dusted with sifted cocoa powder.
Do not be tempted to remove the macarons from the oven tray before they have cooled completely or they will stick.Note