Prepare the Ramekins:
Preheat the oven to 300°F (150°C) and position the rack in the center. Place 4 ramekins in a 9×9 baking pan and set a large kettle of water to boil.
Warm the Cream Mixture:
In a medium saucepan over medium heat, combine the heavy cream, ¼ cup sugar, and a pinch of Kosher salt. Cook, stirring occasionally, until the sugar fully dissolves.
Incorporate the Egg Yolks:
In a large mixing bowl, whisk the egg yolks. Gradually pour in the warm cream mixture, a little at a time, whisking constantly to temper the eggs without cooking them.
Add Espresso Flavor:
In a small bowl, dissolve the instant espresso powder in Kahlua. Stir this into the cream mixture, then strain the custard through a fine mesh sieve into a measuring cup or bowl with a spout.
Bake the Custards:
Pour the custard mixture evenly into the ramekins. Fill the baking dish with enough hot water to reach halfway up the sides of the ramekins. Carefully transfer to the oven and bake for 25-35 minutes (up to 40-45 minutes for deeper ramekins) until the custards are just set. The centers should have a slight jiggle when gently shaken.
Chill:
Remove the ramekins from the water bath and allow them to cool on a wire rack for about 4 hours. Cover with plastic wrap and refrigerate for a minimum of 2 hours, or up to 2 days.
Caramelize the Sugar Topping:
When ready to serve, sprinkle 1 tablespoon of sugar evenly over each custard, rotating to cover the surface. Using a kitchen torch, melt the sugar until it caramelizes and forms a golden-brown crust. Serve immediately.