In my opinion, both Button and ButtonBase controls should have this constructor:

Button(Node graphic)
ButtonBase(Node graphic)

Although the graphic can be set via the method setGraphic(Node value). However, for new people programming in the javaFX language this detail maybe overlooked, or not evident at first perusal. Because at present you have the following constructor:

Button(java.lang.String text, Node graphic)
ButtonBase(java.lang.String text, Node graphic)