>> source: https://www.saveur.com/article/Techniques/saveur-100-beurre-manie/

Simply mix equal parts of @softened butter{} and @all-purpose flour{} together in a small bowl, and use your fingers or a #fork{} to form a smooth paste. Then roll teaspoon-size amounts of the paste into balls.

Note: You can also make large amounts of beurre manie in a food processor and store the balls in the freezer, bringing them to room temperature before use.

When simmering a sauce, whisk in one ball at a time as needed. Allow the mixture to return to a boil, and cook for at east 1 minute to thicken. If your sauce is not as thick as you'd like, add a bit more beurre manie. The butter-coated flour particles will melt and quickly thicken the sauce as it simmers, and the additional butter will add a sleek luster, similar to the effect of mounting a sauce with cold butter.