I have made glaze from the following ingredients:
- 150 g (5.3 oz) melted butter
- 6 tbs powdered sugar
- 1 dl (0.42 cup) cocoa
- 1 dl milk
For the butter I used soft butter that I normally use on bread. I have stored the glaze in fridge for an hour, but it doesn't harden.
Problem solved, had to use real butter.