Techniques
What is an emulsion in cooking?
The short answer. An emulsion is a stable mixture of fat and water, such as a butter sauce, mayonnaise or a vinaigrette.
Fat and water normally separate. Whisking, blending or adding an emulsifier such as egg yolk or mustard keeps tiny droplets suspended in each other.
Rich restaurant mashed potatoes and garlic oil sauces rely on the same fat-and-water idea: butter or oil is beaten or swirled into a little water until it turns glossy and creamy.
Question
Why did my sauce split?
It got too hot or the fat was added too fast. Lower the heat and add the fat gradually while whisking.