Fit the bowl onto the saucepan, making sure the bottom isn’t touching the water. While whisking continuously, add the butter one piece at a time, making sure each piece is completely melted before adding the next, 10 to 12 minutes total. If you are unsure if your sauce is getting too hot, a good way to check is to stick your finger in the sauce. If the sauce is too hot to the touch, then remove it from the heat for a few seconds. When all the butter is added, the sauce should be thickened slightly, enough to coat the back of a spoon.