Put the sugar, cornstarch and kosher salt in a medium, nonstick saucepan, then gently whisk together. Put the milk and egg yolks in a separate pitcher or bowl, then whisk together until combined. Pour the wet mixture into the saucepan with the dry ingredients and stir to combine.
Turn on the heat to medium and cook, stirring gently, until the mixture starts to bubble and thicken, about 10 minutes. When it reaches a pudding consistency, add the butter and stir until melted. Remove from the heat and spoon into bowls. Chill for about 1 hour, until very cold.
Top each bowl with about a tablespoon of salted caramel sauce, some sea salt flakes and unsweetened whipped cream.