Start by melting your butter in a large pot (or Dutch oven), then sprinkle in the flour to make a roux. Whisking constantly until the mixture is thick and creamy. Allow this mixture to cook for 1-2 minutes. Then slowly pour the cold milk into the roux and whisk it constantly. Simmer the mixture, stirring it occasionally, until it thickens. This should take about five minutes. Stir in the spices: paprika, ground chipotle, ancho chile, cayenne, and salt.