May we eat butter on Matzah or is that classified as Gebroks?
The Alter Rebbe writes that Gebroks only applies to Matza with water. Thus there is no issue of Gebroks with Matza and butter.
However, many people will not eat Matzah with Butter either as an extra stringency, or because they can’t be sure no water was mixed into it. Some are lenient to eat it only while the butter is still congealed.
וראה שו״ת שבט הקהתי ד, קלב.