In the past I have had difficulty knowing just how much butter to add to sourdough brioche. I recently got the chance to experiment with a couple of flour to butter ratios at a sourdough class I teach at The Agrarian Kitchen .
This recipe has a total of about 625g flour (500 fresh flour + 125 flour from the starter).Two of the students tried 350g butter, and two other used 300g of butter. Both ratios worked well, with 350g having a richer crumb, but 300g was also acceptable and not too bready to loose that classic brioche texture.
Note that the photos below are from exactly the same recipe here, exept all quantities were multiplied 10 x. The recipe scaled very well!
Cut the butter into 1cm cubes and allow to soften at room temperature for several hours
Mix to combine all ingredients except for the butter, to form a stiff batter (adjust milk quantity if required)
Rest for 10 minutes before continuing to mix for 10 - 15 minutes
The batter should start to resemble a soft dough, but will not have the same degree of elasticity
Massage in the softened butter until no lumps remain
(now is the time to also incorporate dried fruit if you wish)
Place your dough into a covered container
Rise in a cool place, 8C to 15C, for about 15 hours
OR Rise at room temperature until 150% of original size, then refrigerate until the dough hardens all the way through
Remove dough from cool place
Scale and shape into balls to fill small forms (muffin tins are fine) to 75% of the size of the form
AND / OR shape into loaves for larger tins, or specialty forms such as Kugelhopf moulds
Rise at room temperature until 150% to 175% of original size
Blend a full egg and brush gently onto risen shapes
Bake in a moderate oven (215C) for 20 minutes for small brioche, to 45 minutes for larger loaf size brioche