Everyone is telling you too much butter, but if this only happened to part of your batch, it’s more likely you didn’t scrape the sides of your bowl while mixing.

When mixing anything with creamed butter, especially, you need to mix about a third of the time, scrape all the sides and fold the dough, mix, scrape, mix, and do a final scrape/ mix/ fold by hand to make sure all the butter is incorporated.

Stand mixers have this problem more than hand mixers.