Step 1

Melt butter in a medium saucepan over medium-low heat.

Step 2

Add the brown sugar to the melted butter and cook, stirring constantly, for 1 minute (be sure the butter and sugar are completely combined and you can't see any separated butter in the mixture).

Step 3

Bring to a boil over medium heat. Continue cooking, stirring, for 2 minutes.

Step 4

Add the milk and vanilla. Bring to a boil over medium heat. Continue cooking, stirring, for 3 minutes.

Step 5

Remove from heat and set aside to cool. The longer it cools, the thicker it becomes. When it's at your desired consistency, drizzle over the cooled cake and serve.