WoW:API Button SetFont

To set the font for this object please see, API_FontInstance_SetFont.

Instead of using FontInstance:SetFont use MyButtonObject:SetFont. Eg:

 local MyButton = CreateFrame('Button', 'ButtonGlobalName' or nil, ParentFrame or nil, ButtonTemplate or nil)
 MyButton:SetFont(FontPath or nil, FontSize or nil, FontExtra or nil)

Please, read API_FontInstance_SetFont for more exact details.