Use an electric mixer at high speed to whip the butter in a medium mixing bowl until pale and creamy, about 1 to 2 minutes.
Add the molasses and mix until fully incorporated, scraping down the sides of the bowl as needed.
Sprinkle in the flaky salt. Switch to a spatula and fold in the salt by hand, ensuring it is evenly distributed. Transfer the molasses butter to an airtight container or jar and store at room temperature for up to one week.
Notes
Molasses is messy! I highly recommend using a digital kitchen scale rather than measuring cups if you have one.
Scrape down the sides of the mixing bowl as needed.
Molasses butter can sometimes develop crystals. This is normal and delicious.
Storage
Store molasses butter in an airtight container or jar at room temperature for up to one week. For longer storage, refrigerate for up to two weeks.