Butt wink is generally a technique problem caused by squatting back too far or "sitting back" to much. Make sure you only draw your hips back AS you squat down, not before you bend your knees and not too much. Squatting is about bending the knees and staying flat footed. Pulling your hips back just enough to keep your weight centered through your ankles and arches is key. Some people treat sitting back as though THAT is the goal of a squat. But it's not--it's going DOWN!
Now, if your hamstrings are tight that can be addressed. Any stretch where your foot is elevated in front of you on a bench or table will work. Place your heel on the bench, pull your hips back and bend forward at the WAIST, making sure to keep your chest up. Otherwise you'll turn it into a low back stretch which is pointless