Step One: The Roux- In a double boiler, over lightly simmering water, melt butter, stir in flour, blend, and let cook for 10 minutes. Step Two: Heat milk and cream together in a separate pan and stir slowly with a wire whisk bit by bit into flour and butter roux. Add salt and cook slowly for 45 minutes. Variations: An alternate method is to place sauce in a covered ovenproof dish and cook in a 275-degree oven for 45 minutes. For a thinner sauce, more warm milk may be added.