Preheat your oven to 300°F (150°C) and arrange 4 ramekins in a 9x9 baking pan. Boil water for the water bath.

In a saucepan, combine heavy cream, granulated sugar, and kosher salt. Gently heat until the sugar dissolves.

In a separate bowl, whisk the egg yolks until smooth. Gradually mix in the warm cream while stirring constantly.

Stir in the instant espresso powder and Kahlúa (if using) into the custard mixture. Strain through a fine sieve.

Pour the custard into the ramekins. Add hot water to the baking pan until it reaches halfway up the sides of the ramekins. Bake for 25-35 minutes.

Let the ramekins cool for about 4 hours, then refrigerate until ready to serve.

Before serving, sprinkle sugar on top of each custard and use a kitchen torch to caramelize the sugar.