In a heavy-bottom pan, add granulated sugar and water. Heat on medium flame.
Stir gently until the sugar dissolves, then stop stirring and let it boil.
Cook until the mixture turns a deep amber color (caramel forms).
Carefully add butter and whisk continuously (it will bubble rapidly).
Slowly pour in the heavy cream while whisking to combine smoothly.
Add brown sugar, salt, and vanilla extract. Mix well.
Simmer for 1–2 minutes until thick and glossy.
Remove from heat and let it cool. The sauce will thicken as it cools.