dead duck simple.
pot with heavy bottom.
on my smallest gas burner I use a copper plate flame tamer - gives me more temp control.
two sticks unsalted butter aka 16 T aka 8 oz by wt aka 226.8 grams (right,,, go for 225....)
melt butter over low heat; just melt, no brown or bubble.
sprinkle in:
(measured - not guesstimated) 1.5 cups + 3 tablespoons AP flour aka 217 grams (or simply 220 grams - based on KA AP in my kitchen)
stir around/ smooth out / delumpify with a flat wood spatula / implement of choice.
at this point it's a "blonde roux" - very light color - altho even for use as a blonde it needs to cook a bit to eliminate the raw flourly taste....
bring up heat and stir regularly to constantly - the heat "toasts" the flour and the color goes to light to medium to dark(er) brown.
this should take 10-15 minutes of stirring pot tending. which is why I do this in the idle moments ahead of time . . . do not rush it. too high a heat and momentary no stir will get you burnt flour. which tastes terrible, throw it away if it burns - do not try and "save" it.
let it cool on the counter, then in the fridge overnight. when ready, remove from fridge, give it 5-10 minutes to warm _only_ slightly, then using a wide wooden spatula split/cut it down the middle - each side should tilt up at the pan edges. flip the pan over and dump out the (two halves) thick pancake of brown roux. cut into squares/chunks with a knife - into a bag and into the freezer.
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notes from the mad kitchen....
(1) I use unsalted butter - only because one never know where one's roux is going to end up and one can always add salt but removing it is trickier....
(2) I use a stainless interior saute pan with straight sides. non-stick can be used as the temps really don't get very high.
(3) a wide wood spatula is ideal for stirring - it will be thick soupy when hot.
(4) the ratio of flour to butter is not set in stone. minimum is 1:1 - the 3:5 ratio above is "classic French" - less flour = thinner consistency when hot and also less thickening per cube when used.
(5) the darker you take it, the less thickening power it will have.
it keeps near forever in the freezer. I prefer small cubes vs big chunks as one can always put "another cube in the barbie" when thickening the "whatever"
I use it frozen - right out of the freezer - it melts in the "whatever"
I typically don't do a blonde roux for use in a white sauce, for example, as that goes pretty quick "on demand" - but to develop the color and the toasty flavor that comes with the color takes time and stirring - which is the advantage of having some "pre-made"
and, if you're doing something that needs a darker color, a couple drops of Gravy Master when nobody is looking does the trick in splendid fashion....