When I looked it up fawn was listed as A- B- C- dd ee and black of course is aa B- C- D- E- . So if we assume the second gene represented by the - is only dominant with no hidden recessives we are crossing AA BB CC dd ee with aa BB CC DD EE and every offspring will get one A , one B, one C, one d, and one e from the fawn and one a, B, C, D, and E from the black resulting in Aa BB CC Dd Ee or 100% chestnut. Unless you are referring to another genotype as fawn.
Black and agouti in rabbits does keep confusing me since my genetics classes were on horses and black/red is E and e in horses with the agouti gene A being bay or not bay and having no impact on ee genotypes.