Measure all the ingredients before beginning. Finely chop the dark chocolate if you're using a bar.

Prepare an ice bath by filling a large bowl with ice and cold water. Place a second mixing bowl nearby, making sure it will comfortably fit inside the ice bath.

Add the 140g water and 30g espresso to a small saucepan.

Bring the coffee and water mixture just to a boil, then immediately remove it from the heat.

Place the chopped chocolate in the mixing bowl. Pour the hot coffee mixture over the chocolate.

Stir gently until the chocolate is completely melted and the mixture is smooth and glossy.

Carefully set the bowl containing the chocolate mixture into the prepared ice bath.

Begin whisking continuously. As the chocolate cools, it will gradually thicken, become slightly lighter in color, and take on an airy consistency.

Continue whisking the Dark Chocolate Coffee Mousse until the whisk begins leaving ribbons across the surface. The consistency should resemble softly whipped cream.

Immediately remove the bowl from the ice bath. Don't continue whisking once the ideal texture has been reached, as the mousse can become too firm.

Spoon or pour the mousse immediately into small ramekins, dessert glasses, or serving cups.

Serve when softly set, or refrigerate briefly for a firmer consistency.