When your title says "mottled," do you mean specifically black with white dots? Or do you mean any color with the mottling gene?
Any of those could be useful. You will definitely need more crosses with Marans at some point to get more of the genes for dark brown eggs.
If Marans have the right size and dark brown eggs, and d'Uccle have the other traits you want, then I don't see why you have Faverolle in the mix at all
If you want to work with the cockerel you have now, maybe breed him back to d'Uccle. Otherwise just start with Marans x d'Uccle.
From there, I would probably follow a two generation breeding plan:
--breed a mottled bird to a Marans. The first "mottled bird" will be a d'Uccle, but later it will be a bird from your own breeding. All of the chicks will carry mottling but none will show it. Choose one or more cockerels that have as many d'Uccle traits as you can get (muffs, feathered feet, etc), and if there is variation in size you want to keep the biggest ones.
--breed cockerels (previous paragraph) to a mottled bird (d'Uccle the first time, but later this will be a bird you have bred.) About half of chicks will show mottling. Among the mottled pullets, pick ones that have all the other d'Uccle traits (muffs, feathered feet, etc). Try to pick big ones if you can, but the other traits are more important than the size in this generation. Then raise up those pullets and see what color eggs they lay. Choose the ones who lay the darkest brown eggs, and use them as the "mottled" birds in your next generation (breed these mottled birds to Marans, then a son back to them, then pick pullets from that to breed back to Marans again...)
The reason I suggest this:
Each cross to Marans will give the genes for large size and dark eggs. Each cross back to a mottled bird will give some birds that show mottling (recessive gene), so you can be sure you don't lose that trait. You can see what color eggs pullets lay which is why you pick dark-laying pullets in that generation. You cannot see what color eggs a cockerel will lay, but if he had a Marans parent you can trust that he has at least some genes for dark eggs, which he will give to at least some of his daughters (and you need to keep a cockerel not a pullet in that generation, because you will be breeding back to his mother and her sisters-- who are obviously female).
Or if you think Faverolles have all the traits you want (except mottling and egg color), maybe cross Faverolles with a larger mottled breed (like Ancona or Speckled Sussex), then backcross to the mottled breed to get some that show mottling as well as the muffs and feathered feet. Use those chicks as the "mottled" birds for crossing with Marans to get dark eggs. (Or you could crossing your current cockerel to a mottled hen of some large breed, pick some mottled chicks with the right other traits, and go from there.)
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Alternative breeding strategy:
Start with a d'Uccle/Marans mix or with your current cockerel, and breed back to Marans. Half of chicks will carry mottling but not show it, half will not even carry it. Choose chicks that show all the other d'Uccle traits you want (muffs, feathered feet, etc), and test-mate them with a d'Uccle to learn which ones carry mottling. Take one that carries mottling and breed back to Marans again. Repeat for as many generations as it takes, then breed some of them to each other to get about 25% chicks that show mottling.
This works by crossing repeatedly to Marans to get the dark eggs and large size faster, and you can see muffs & feathered feet so it is easy to be sure you have those genes, but you will probably lose the mottling gene unless you test-mate each bird to be sure it carries mottling before you use that bird to cross to Marans.
If you can hatch chicks only once a year, having to test-mate birds will make this the slower strategy. But if you can do a test-hatch and then the actual mating within the same year, this strategy will probably end up with the right results faster. Raising more than one generation per year will also speed things up. Using males of your mixes, bred with Marans hens, will be faster than using Marans roosters with mixed pullets. (Partly because cockerels can be siring chicks before their sisters start laying, partly because females can retain sperm for a while after a mating so after test-mating a pullet you would need to wait before hatching eggs from her with a different rooster.) But of course you will need to raise females at some point to check on the egg color, and to breed with the males to get a pure-breeding line of birds.
I don't know how much you are in a hurry (one generation per year vs. more), and which traits you want to see fastest in your project birds (first strategy will have you working with more birds that show mottling, second will have you working with more birds that produce dark brown eggs). I described two different breeding plans, but you can also bounce back and forth. A bird that carries mottling (because one parent shows mottling or because you test-mated it to tell) can be bred back to a mottled bird to produce some chicks with mottling, or it can be bred to a Marans to produce some chicks that carry mottling and some that do not (but you can tell which is which by test mating them.)
Any method will produce many chicks that are "culls," meaning you do not want to breed from them on this project. If you do not already know what you want to do with the culls, I suggest you think about it before you hatch very manymore chicks. You might be able to sell some to other people, keep some females as layers, butcher males and extra females even if they are small, etc.
