A basic butter sauce for pasta starts with mixing 2 tsp of cornstarch with about 2 Tbsp water till well combined. Pour this into 1 cup of water or milk and set aside. Cut 3/4 cup of butter into half-inch cubes. Melt these over medium-low heat, and stir in the cornstarch-water mixture. Bring to a low boil, stirring gently and continuously for up to 5 minutes. Butter is an emulsion of fat and water, so this sauce needs low, slow cooking to keep that bond intact. In general, use 1 Tbsp of cornstarch for each cup of liquid you want to thicken. Always mix cornstarch with a small amount of liquid before adding to the rest of the liquid to prevent lumping. Use lower heat for cornstarch than you would for flour. Cornstarch is not as effective for thickening acidic sauces made with lemon, tomato, or vinegar. The powdered sugar in buttercream frosting already contains cornstarch, but if your frosting seems too soft add 1/2 tsp cornstarch per cup of sugar to soak up extra moisture. This was more info than you asked for, but better to know it than not.
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In general, use 1 Tbsp of cornstarch for each cup of liquid you want to thicken. Always mix cornstarch with a small amount of liquid before adding to the rest of the liquid to prevent lumping. Use lower heat for cornstarch than you would for flour. Cornstarch is not as effective for thickening acidic sauces made with lemon, tomato, or vinegar. The powdered sugar in buttercream frosting already contains cornstarch, but if your frosting seems too soft add 1/2 tsp cornstarch per cup of sugar to soak up extra moisture. This was more info than you asked for, but better to know it than not.