As Jo said, butter is used in some commercial ice creams (I assume when it's more readily available than fresh cream).. When you make butter, you break the cream's emulsion and all the fat globules coalesce. To use butter in ice cream and to get the best texture, the fat needs to be broken up into the smallest globules possible and re-emulsified. Ideally this is done with a high-pressure homogenizer and strong emulsifying ingredients.
If you're just using butter as a flavor source (brown butter ice ice cream, etc.) and there's also a decent amount of cream in the mix, it's not too challenging to get good texture. Make sure it's an egg-based recipe or that you're adding emulsifiers, and ideally, blend the bejeezus out of the hot mix with a vitamix or equivalent.
