Now that the sauce has cooled slightly***, add the butter, cream, cilantro/fenugreek leaves, and final tsp of garam masala and stir until well incorporated. Sauce should be thick enough to coat the back of a spoon.
***When the sauce is very hot, the cream and butter will not thicken the sauce substantially. If that happens, just put it in the fridge for a few minutes and it will thicken up.