Combine sugar and water in a medium saucepan. Stir gently until sugar dissolves. Avoid splashing. Turn the heat to medium and let it boil slowly for caramelization.

Let the sugar-water mixture cook without stirring. Watch carefully as it turns golden brown. Swirl the pan gently if needed for even coloring.

When the mixture turns deep amber, remove the pan from heat. Dark caramel has a slightly smoky, rich aroma. Do not wait too long or it may burn.

Slowly pour heavy cream into the caramel. It will bubble violently, so pour gradually while stirring with a whisk. Keep whisking until smooth and creamy.

Add butter to the caramel mixture. Stir until melted completely. Butter gives richness and a glossy texture to the sauce.

Add salt and vanilla extract. Stir well to combine. This balances sweetness and enhances the deep caramel flavor.

Let the caramel sauce cool for a few minutes. It will thicken slightly as it cools. Pour into a jar for storage or use immediately.