Preheat oven to 325°F (160°C). Arrange six ramekins in a large baking dish.

Warm milk, heavy cream, and chopped chocolate in a saucepan over medium-low heat, stirring until chocolate is fully melted and mixture is hot.

Whisk egg yolks, sugar, and salt in a bowl until smooth. Temper eggs by slowly whisking in about ½ cup of the warm chocolate mixture.

Pour tempered egg mixture back into the saucepan with remaining chocolate. Whisk in espresso powder and vanilla extract until slightly thickened, coating the back of a spoon.

Divide custard evenly among ramekins. Place baking dish in oven, then carefully add water to the dish until it reaches halfway up the sides of the ramekins.

Bake for 50-60 minutes, until centers are just set but slightly wobbly.

Remove ramekins from water bath, cool completely on a wire rack, then refrigerate for at least 2 hours until fully chilled.