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Chocolate seems like quite the challenge, since you need to keep it cold, but if you cool it too much it will “bloom” and look bad. Not a lot of margin for error there. I’ve never done this, so all I can do is say what I would do if I were trying this!
I don’t know what tool you are using, but I expect that a sharp single-flute endmill with aggressive rake and large relief would work best. I’d think about it like plastics that soften easily when cut, like polycarbonate. If you use a bit optimized for polycarbonate it might work.
A slower spindle speed (larger chip load) might help keep the chip size larger, improving chip evacuation and reducing heating and smearing; you want chips not dust I think. I would think you would want visible curls of chocolate rather than dust. You might want chip load of ¼ of your tool diameter or so — slightly less than the flute depth. I would also expect that I’d want at least ¼ my tool diameter for depth of cut in each pass, and I would experiment past ½ and take the deepest passes that worked.
What are you using to evacuate the chips? Vacuum? Compressed air?
How cold is the chocolate, and how are you keeping it cold?