Work two tablespoonfuls butter into two cupfuls powdered sugar, moistening with boiling water, beat hard for a few minutes and put the vessel containing it in a saucepan of water which is at a hard boil. Stir until scalding hot, put in a teaspoonful cornstarch moistened with cold water, boil up once, add a glass of good brandy, and remove from the fire.
If you wish it still thinner than this use half the cornstarch called for or omit it entirely.