Combine the instant espresso powder with the condensed milk. Set it aside for now so that the instant espresso dissolves in the condensed milk.
Using a hand mixer or stand mixer, whip the heavy whipping cream to stiff peaks.
Beat the condensed milk and espresso powder for about a minute with the hand mixer to make sure the espresso is dissolved and there are no lumps.
Fold 1/3 of the whipped cream into the espresso-condensed milk mixture with a silicone spatula. Then, fold another 1/3 carefully. Finally, fold the last third, gently folding with the spatula so as not to knock the air out of the whipped cream.
Transfer into serving glasses or containers and refrigerate until the mousse is set (preferably overnight but for at least four hours). You can go for cocktail glasses, dessert glasses, or ramekins, depending on what you have at home and what you’d prefer.
Finally, for an optional finishing touch, serve the mousse with toasted almond flakes and/or chocolate sprinkles. The nuts add a bit of texture to the soft mousse and the chocolate pairs well with coffee.