In a small saucepan combine the butter, brown sugar, and cream. Bring to a low boil, stirring continuously and cook until the brown sugar is dissolved.
Remove from the heat and stir in the vanilla, Let cool for a few minutes.
Add the powdered sugar slowly, whisking after each addition, adding until you have reached the desired consistency for the glaze.
For a cake topping, it should be a thicker glaze that you spread on top. For drizzling over scones and muffins you need a slightly thinner glaze.
Use immediately as the icing will set within minutes.
Notes
If you need to thin the glaze ( eg. you added too much powdered sugar) simply add more cream at the end, teaspoon by teaspoon, until you reach the desired consistency.