If you could land a partridge brahma male, breeding him with your dark brahma hen would give you your best start, short of also buying a partridge brahma hen.
I was playing around with Henk's chicken calculator. Crossing your buff brahma male to your dark brahma hen should produce "black patterned yellow/golden columbian" cockerels. This cockerel breed back to the dark hen should produce 16 possible slightly different genetic color combinations, a couple are getting in the neighborhood. Keep breeding the ones that most closely resemble the partridge pattern will get you close. I don't think you will ever get them dark enough without bringing in the mahogeny gene, if that is a concern for you. Going this route would save you the trouble of refixing traites like comb, foot feathering, and type to a certain degree.
Proper brahma type is very important to keep in mind in a project like this. There should be no doubt to any one who sees your birds, that you are raising brahmas, the color is secondary. That being said, you try to use the typiest birds with the closest color to what you trying to get in your breeding pens.
You main cost will be feed and facilities. You will have to hatch a good number of chicks to get the best chance of producing the chicks that will help you in you project. Brahmas are a long time growing up. Some things are apparent fairly early, but you will still need to grow out a good number to give you the best chance of choosing proper type and color in next years breeders. Believe me, and mine free range too, they will eat a lot of feed.
good luck
big medicine
