Preheat the oven to 300°F. Place 6 individual gratin dishes on a large, high-rimmed baking sheet.
In a small saucepan whisk the cocoa powder with a little bit of cold milk until fully incorporated.
Place the saucepan over medium heat, warm the cream, milk, sugar, chocolate and salt until the sugar dissolves and chocolate is melted, which shouldn’t take long at all.
In a bowl, whisk together the egg yolks.
Slowly add the warm chocolate mixture to the egg yolks in a steady stream, whisking continuously until the chocolate mixture is completely incorporated. Mix in the espresso or coffee powder and strain the mixture into a large measuring cup. Divide the mixture among the 6 dishes.
Place the pan of custards on the oven rack and pour enough hot water into the pan so that it reaches at least halfway up the sides of the dishes.
Bake the custards for 20 to 25 minutes, or until they barely jiggle in the center. Start checking them around the 20 minute mark and remove them before they are too firmly set (no jiggling at all).
Remove the dishes from the pan and set them on a cooling rack until they cool to room temperature. Cover and refrigerate until ready to serve.
Just before serving, sprinkle the top of each crème brûlée with an even layer of sugar. I use just enough to evenly cover the tops, particularly when using gratin dishes as they have a lot of surface area and you don’t want overdo it on the sugar.
Using a blowtorch, melt the sugar coating until it begins to brown. Serve immediately.