Place heavy-bottomed saucepan on stovetop over medium heat. Add 1 cup unsalted butter and let butter melt completely.
Once butter is melted, sprinkle 1 cup all-purpose flour into saucepan. Whisk flour into melted butter until fully incorporated with no lumps remaining.
Cook roux, whisking constantly, approximately 10 to 15 minutes or until roux begins to develop rich blonde color.
Reduce heat under saucepan to medium-low. Continue cooking roux, whisking constantly, 5 to 10 minutes, or until roux develops rich peanut-butter color.
Reduce heat under saucepan to low. Continue cooking roux, whisking constantly, 10 minutes or until roux is rich, dark brown color, resembling melted milk chocolate.
Turn off heat under saucepan, but do not remove saucepan from burner. Continue whisking, cooking roux until color is deep, dark brown, resembling melted dark chocolate.
When desired color is achieved, immediately add ingredients to continue making gumbo. Alternately, transfer roux to airtight container. Let roux cool completely, then seal container and refrigerate or freeze roux until ready to use.