I don't get why you put part of the butter in the flour?

Hello! It's sort of like a fake beurre manie—French term for a mixture of flour and butter, added to sauces, soups, stews, etc to help thicken them. While a beurre manie is usually equal parts of flour and butter, kneaded together, I wanted just a little flour (for a thicker, but not too thick, sauce). So, by coating the butter pieces in flour, we can thicken the sauce while ensuring that the flour disperses evenly and doesn't clump.