1. Place chocolate and butter in bowl over a saucepan of simmering water until melted. Remove from heat and stir in sugar. Add eggs and vanilla and beat well.
2. Sift the flour, cocoa and baking powder into a large bowl. Add the chocolate mixture and stir well. Cover with plastic wrap and chill for 1 hour.
3. Preheat oven to 160C. Line a baking sheet with non stick paper.
4. Place icing sugar in a bowl. Form heaped teaspoons of dough into balls and roll in icing sugar.
5. Place on tray, slightly flatten and bake in oven for 10-15 minutes until tops of biscuits rack.
6. They will be soft when removed, but will harden upon cooling. When cool, sandwich icing between two biscuits and serve.