In a small bowl, whisk espresso powder into 1 tablespoon hot water and set aside to cool.
In the bowl of a stand mixer or in a large bowl with a hand mixer or whisk, beat egg whites with cream of tartar until soft peaks form, about 2-3 minutes.
Slowly add in granulated sugar, a spoonful at a time, beating until glossy stiff peaks form, about 2 minutes. Place meringue in the fridge while whipping cream.
In a second bowl of a stand mixer or in a large bowl with a hand mixer or whisk, beat heavy cream with confectioners' sugar, vanilla, and salt until soft peaks form, about 2-3 minutes.
Gently stir cooled espresso mixture into whipped cream until streaks disappear.
Fold about ⅓ of the meringue into the whipped cream to loosen it. In two batches, gently fold in the remaining meringue, keeping as much air in the mixture as possible.
Divide mixture between 4 small glasses or ramekins and chill for at least 4 hours or up to overnight.
Before serving, sprinkle coffee mousse with cocoa powder and top with whipped cream, if desired.