I don't want to get between a Frenchman and his butter, but I just use a potato peeler for cold butter. I find it works well enough. I've also used a cheese slicer, but I found it harder to control the thickness.
Or simply use a regular knife, either the teeth or the back, depending on the type of cut you want, to scrape the slab's surface.
I love Japanese strive for perfection and their tools, but I feel butter cutting is a solved problem.