Dissolve the espresso powder. In a small bowl, dissolve 1 tsp instant espresso powder in 1 Tbsp pure vanilla extract with a whisk or fork.
Melt the chocolate. Place 12oz (340g) chopped semi-sweet chocolate and 1½ cups heavy cream in a heatproof bowl set over a pot of simmering water. Cook, stirring occasionally, until the chocolate is melted and the mixture is completely smooth.
Combine egg yolks and sugar. In a separate large bowl, whisk together 6 large egg yolks with 2 Tbsp granulated sugar until the mixture turns a pale yellow color, about 2-3 minutes.
Add the chocolate. Slowly add a little of the chocolate into the egg mixture, whisking the entire time. Once the eggs are tempered, you can pour in the remaining chocolate and cream mixture along with the vanilla extract and ¼ tsp kosher salt. Set in the fridge to cool.
Beat the egg whites. Place 6 egg whites in the bowl of a stand mixer, then whisk until soft peaks form. With the mixer on low, slowly pour in the remaining ⅓ cup sugar, then continue whisking until the egg whites are glossy and have stiff peaks, about 5 minutes.
Fold together. Carefully fold the egg white mixture into the cooled chocolate mixture in 3 additions, being sure the egg whites are fully incorporated before adding more. Try not to deflate the egg whites as you fold them into the batter.
Refrigerate. Transfer the mousse to individual ramekins (or one large bowl), then cover tightly with plastic wrap. The plastic should not touch the top of the mousse. Refrigerate for at least 4 hours or until the mousse has fully set, preferably overnight.
Make homemade whipped cream. Place 1½ cups (355ml) heavy cream, 3 Tbsp (6g) powdered sugar, and 2 tsp vanilla extract in the bowl of a stand mixer. Whisk on medium-high speed until medium peaks form. If you prefer stiff peaks, beat it a little longer.
Serve! Spoon some of the chocolate mousse into individual serving bowls, then top with a little homemade whipped cream and any additional toppings you like. My favorite way to serve it is with flaky sea salt sprinkled on top. Enjoy!