In a large skillet over medium high heat, cook the bulk sausage until brown and crumbly.
When the sausage is done, add the butter if there is little to no residual butter in the pan. If there is excess oil for the sausage, you may not need to add butter. Ideally, you want about 3 tablespoons of fat, either from the sausage or from the melted butter.
Sprinkle sausage with flour, stir, and allow to cook for several minutes until the flour can absorb some of the fat and get golden brown.
Add the milk and stir. Continue stirring occasionally over medium heat until gravy thickens, 5-10 minutes.
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Notes
Sausage: You can buy in bulk from the meat counter at the grocery store, you can buy links and remove them from the casing, or you can make your own homemade breakfast sausage from ground pork and seasonings.
Keep it warm: You'll want to serve the gravy hot. Because it's a cream based gravy with a fair amount of fat, it will get thicker as it cools. To keep warm, just heat over low heat on the stove until ready to serve.
To store: Keep the extra gravy in an air tight container in the refrigerator. Should keep for 3-5 days.