Butter is an emulsion, a tenuous mixture of water and fat, and prone to separation when exposed to heat. This is a big deal when thickening garlic butter sauce. You can't bring butter sauce to a simmer like regular sauces, so gelatinizing starch thickeners take about twice as long as usual. You must also heat the sauce gently and add the thickening agent in the last minute or so of cooking it. There are quite a few options such as monter au beurre, which is whisking in cold butter to the garlic butter sauce or adding pre-gelatinized flour to the sauce to lessen the amount of time required compared to using a gelatinizing starch thickening agent. Whether whisking in additional cold butter to the existing sauce or using one of the following solutions, these tips are all easy and utilize ingredients that should already be in your kitchen.
Using a Liaison to Thicken
A liaison is perhaps the most elegant thickening agent, or at the least the one with the most finesse. A classic French thickener, a liaison is a mixture of egg yolks and cream that will thicken garlic butter sauce slightly without any starchiness or change in color. The sauce stays golden and smooth.
Mix one egg yolk and two ounces of heavy cream, for every one cup of sauce, in a measuring cup or mixing bowl until smooth and uniform in color. Add about half of a cup of hot garlic butter to the liaison and whisk to blend. Next, add the mixed liaison to the pan of garlic butter sauce. Cook the butter garlic sauce over low heat for four to five minutes, stirring frequently.
Garlic butter sauce thickened with flour essentially makes it a roux-thickened sauce. You typically have to simmer roux-thickened sauces for about 10 minutes to gelatinize the starch granules and produce a smooth texture. With garlic butter sauce, however, you have to gelatinize the starch over low heat for about twice as long, or until you can't detect any trace of grittiness.
For every one cup of garlic butter sauce, add one tablespoon of flour. Over low heat, whisk the sauce enthusiastically to incorporate the flour. Cook the sauce for 15 to 20 minutes or until it has a smooth texture when you taste it.
Using Cornstarch as a Thickening Agent
Cornstarch thickens relatively quickly, so you don't have the textural issues you can have with flour. But you have to dissolve cornstarch in water before adding it to the sauce to prevent the cornstarch from creating clumps in the sauce. Remember to heat the cornstarch and water mixture for several minutes to cook off the water.
For each cup of garlic butter sauce, mix one tablespoon of cornstarch in one tablespoon of cold water. Whisk the cornstarch into the butter. Cook the sauce over low heat until thickened, which should require about five to 10 minutes on the stovetop.
Tips for Small Quantities
If you have a small quantity of garlic butter sauce, such as the amount used for one serving of shrimp scampi or pasta, sprinkle half a teaspoon of flour over the surface and set the heat to low. Whisk the flour to blend and let it cook over low heat for three or four minutes before serving.