In a medium saucepan, melt butter over medium
Add flour and salt; cook, stirring frequently, until mixture is pale golden, has a slightly nutty aroma, and is the texture of thick wet sand
Whisking constantly, pour in 1/2 cup chicken stock until smooth and then add the last 1/2 cup of stock and whisk until smooth again
Add 2 cups milk and cook mixture, stirring constantly along the bottom of pan, until boiling, about 5 minutes
Reduce heat to low, simmer and continue stirring occasionally until the sauce thickens and coats the back of a spoon
Add the sherry or Madeira and stir to incorporate
Remove the saucepan from the heat and allow to cool slightly