Place the granulated sugar and water in a small saucepan and cook over medium heat without stirring until the syrup turns a deep amber color.
Immediately divide the caramel among six 6-ounce ramekins, tilting each ramekin so the caramel covers the bottom. Set aside while the custard is prepared.
Whisk 1/2 cup of the whole milk with the cornstarch and agar powder until completely smooth. Add the remaining milk, heavy cream, sweetened condensed milk, and fine salt.
Cook the custard over medium heat, whisking constantly, until it reaches a full simmer and thickens. Continue simmering for 2 minutes so the agar fully activates, then remove from the heat and stir in the vanilla extract.
Pour the hot custard evenly into the caramel-lined ramekins. Cool at room temperature for 20 minutes, then cover and refrigerate for at least 6 hours until fully set.
To unmold, dip the bottom of each ramekin in warm water for 8 to 10 seconds. Loosen the outer edge with a thin flexible spatula.
Place a dessert plate over each ramekin and invert in one smooth motion. Let the caramel sauce run over the custard before serving.
