I use corn starch & 1/2 and 1/2 to make my cannoli cream. I use a double boiler. My recipe calls for a lot of stirring up to an hour in order to get the thickness I need to stuff cannoli shells. My question is that the mixture never comes to a boil, so what am I doing wrong? & is it necessary to stir for so long? I look forward to your answer. thank you Joann
Without knowing the exact recipe, it is difficult to give a precise answer, but my initial thoughts are that you have an issue with low temperature.
For cream to thicken you need to either a) Evaporate a lot of water from the cream itself or b) Use a thickening agent such as flour or cornflour. Both these approaches need higher temperatures than are generally achieved with your average Bain Marie (65 - 100C depending on the materials used), with cornflour thickening at 95C and flour at boiling point. While you will achieve some evaporation at lower temperatures, it will take forever.
Different materials have widely different efficiencies when used as double boilers. I'd check the temperature of your mixture, and maybe consider using a stainless steel bowl rather than glass, plastic, ceramic or Pyrex which will improve the heat conductivity a lot. Alternatively, you might need a different bowl / pan setup, as a bowl in a large saucepan (a long way from the simmering water) may not reach temperature.
If low temperature is crucial to your recipe, you could substitute gelatin as the thickening agent, that thickens down to 35C.