I've never bio'ed the butter in between. It takes way too long, and you'd have to measure out an exact amount to consider a 'dose'.
You can bio it when the whole batch is done, determine the dose, and then back-calculate how much you will need in the recipe if say it makes 10 brownies you want 10 doses, and then you calculate how much POT butter and REGULAR butter you have to mix to get close to that number.
Remember it's easier to dilute the butter later than have to play with the recipe to allow for WAY more butter just to hit one dose.