Melt butter in a saucepan over medium-low heat. Gradually whisk in flour, reduce heat to low, and stir until the roux has turned golden and smells like a cooked pie crust, about 10 to 12 minutes.

Whisk in about 2 cups of cold stock, a little at a time, stirring to remove any lumps. Add remaining 2 cups stock.

Cook, stirring occasionally, until gravy is thick enough to coat the back of a spoon, about 10 to 15 minutes.

Stir in heavy cream, and season with salt, white pepper, and cayenne. Taste and if the gravy still tastes starchy, let it simmer a little longer, adding more cold stock if necessary.