Mix sugar and flour together in a sauce pan, blend well. Add the boiling water and the salt. Cook over medium heat, stirring constantly, for 6-8 minutes, or until it thickens and starts to come away from the edges of the pan. If it is too thick, add a little more water. Remove from heat and add the butter, the cream, and the vanilla (I used about 1/2 tsp.). Stir until thoroughly combined.