Jars are a fun way to tote and serve the mousse, but you can use custard cups, ramekins or even teacups instead.
1. In a small bowl, sprinkle gelatin over 2 tablespoons water. In a medium bowl, whisk together egg yolks, granulated sugar and a pinch of salt until pale, about 1 minute.
2. In a small saucepan, heat 1 cup cream and espresso powder over medium until warm, stirring occasionally with a silicone spatula. Gradually whisk warm cream mixture into egg mixture until combined, then return to saucepan. Stirring constantly, cook over medium heat until mixture thickens, about 8 minutes. Transfer to a bowl and whisk in gelatin mixture. Press plastic wrap onto surface of espresso mixture and let cool completely (do not refrigerate), about 45 minutes.
3. In a large bowl, using an electric mixer, beat 1 cup cream until soft peaks form. With a spatula, gently fold cream into espresso mixture. Divide among six 4-ounce containers. Cover and refrigerate until set, 1 hour (or up to 1 day). Before serving, whisk 1/2 cup cream and confectioners' sugar until soft peaks form. Top mousse with whipped cream and sprinkle with cocoa powder, if desired.
Cook's note: Make the mousse up to a day ahead. Take heavy cream in a jar to whisk up at the party. A shaker makes cocoa powder a cinch to sprinkle.