Combine mustard powder, cold water, and salt in a small bowl.
Stir until the mustard powder is fully hydrated and the mixture is smooth.
Let the mustard mixture sit briefly to allow the mustard to activate and develop its sharp, spicy flavor.
Add the mustard mixture to the softened butter.
Mix thoroughly until the mustard is evenly distributed throughout the butter.
Spread generously onto warm breakfast sandwich bread, biscuits, English muffins, or toast.
Store leftover butter in an airtight container in the refrigerator.
