Prepare while doughnuts rise.
Measure 1 cup granulated sugar into a heavy pan and stir and melt slowly over low heat until syrupy.
Add 1 cup boiling water and stir until syrup melts completely.
Measure 2 cups icing sugar into a small bowl and stir in enough caramel liquid to make icing the consistency of thick cream.
Store remaining caramel liquid for future use.