Add the first 6 ingredients to a small sauce pan and heat over the stovetop on medium-high heat, stirring until the red chili paste and brown sugar dissolves. Bring to a boil and reduce the sauce by half, or until lightly syrupy. Meanwhile, with a fork, mash the butter and corn starch together (this is in the fashion of making a Beurre manié). Once sauce has reduced, turn heat down to low, and whisk in the butter cornstarch mixture until thickened. If the sauce is too thin, mash together another tablespoon of butter with 1/2 teaspoon of cornstarch, and add to sauce, repeating for desired consistency. Likewise, if the sauce is too thick, whisk in water, one tablespoon at a time. Add freshly ground pepper to taste, and keep over low heat until ready to serve.