1.Beat butter, sugar and egg in small bowl with electric mixer until combined. Stir in sifted flours and cocoa in two batches.
2.Knead dough on floured surface until smooth; roll dough between sheets of baking paper until 5mm thick. Cover; refrigerate 30 minutes.
3.Preheat oven to 180°C (160°C fan-forced). Grease oven trays; line with baking paper.
4.Using a 8.5cm shoe-shaped cutter, cut 25 shapes from dough. Place about 3cm apart on trays. Bake about 12 minutes. Cool on trays.
5.To make coloured sugar; divide sugar among four small plastic bags. Add different colouring to each bag to tint sugar. Rub colouring into sugar until combined.
6.Spread cookies with chocolate. Sprinkle shoes with coloured sugar; decorate with cachous.