Butter an 8" x 8", 2.2 Q or 2L pan or line with parchment paper for easy removal.
Add the Dulche de Leche, brown sugar and butter to a 2 quart microwave safe bowl. Microwave on high for 10 minutes, stirring every 2 minutes.
Let cool for 5 minutes, add the vanilla and beat with electric mixer for 3 minutes until smooth.
Pour into prepared pan, sprinkle with coarse salt, chill for at least two hours, cut into squares.