Melt the butter in a medium sauce pan on low to medium heat.
Sprinkle in the flour and whisk until it thickens creating a roux, about 1-2 minutes.
Add about 1/2 cup milk whisking continuously until milk is absorbed and then add remaining milk. Continue cooking and stirring until bubbles start to appear.
Add cheese and taco seasoning to the roux and stir until mixture is melted and combined.