Butter Cookie is a slur used to describe Danish in a derogatory way. Reference to Kjeldsen's Butter Cookies, a fairly famous export from Denmark.
The word Butter Cookie is a racial slur that has been used historically to degrade and dehumanize danish. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to danish. If you are unsure about whether or not it is appropriate to use the word Butter Cookie, it is best to err on the side of caution and avoid using it in any situation.