Heat a cast iron skillet over medium heat and add the fat.
Add the flour and heat over medium heat, whisking constantly.
Continue stirring until desired color is reached. Less than 5 minutes for white, 5-10 minutes for blonde, 15-25 minutes for medium brown and 30-40 minutes for dark brown. (See additional details in the post above)
Notes
Many different fats can be used to make this recipe. Try using butter, oil, duck fat, goose fat, ghee, coconut oil, canola oil, or vegetable oil.
When making a dark brown roux, I don't recommend using butter. Butter has milk solids in it when have a tendency to burn if cooked for an extended amount of time. Any other higher smoke point fat works great for a dark roux. I personally use vegetable or avocado oil.
Roux should be whisked or stirred continuously while cooking to avoid burning your roux. If you see any black specks or dots, it means the roux has started to burn and you will need to throw it out and start over.
When making a dark roux, it will go from a dark chocolate color to burned very quickly - make sure to pay attention and stir constantly.