• Set up the double boiler: This recipe requires you to heat multiple ingredients in a double boiler situation, so you'll need two separate medium heatproof bowls that can rest atop a medium saucepot with a couple of inches of simmering water such that the bottom of the bowls do not touch the water.

  Set your saucepot with water over medium-low heat. Bring to just below a simmer (no more than 190℉/87℃).

  Separately, fill a large bowl with cold water and set aside.
• Whisk the egg yolks: While the water heats up, vigorously beat 4 large egg yolks and 150 grams caster sugar in a medium bowl until the mixture has thickened significantly and turned pale yellow, about 3 - 4 minutes.

  Note: If ready, when you lift the whisk, it should flow off in a ribbon that sits on the surface for a few seconds before dissolving back into the bowl. This step is critical to developing a slightly "chewy" mousse texture.
• Heat and cool the yolks: Whisk in 3 tablespoons orange liqueur, then set over the saucepot and continue whisking for 3 - 8 minutes until the mixture is foamy and too hot for your finger to touch, about 178℉/81℃.

  Turn the heat off (but do not discard the water, you'll need it again), and place the bowl with egg yolks over the bowl of cold water, and continue whisking again until the mixture has fully cooled to room temperature and thickened once again into that ribbony texture. Set aside.
• Melt chocolate: In another medium bowl, add 170 grams high-quality bittersweet chocolate, 1 ½ teaspoons espresso powder, and 60 ml hot water. Bring the saucepot back to a bare simmer, and place the bowl over the pot until the chocolate is fully melted, about 2 - 4 minutes.

  Remove from the heat, then add 170 grams unsalted butter, whisking until fully incorporated. Mix the melted chocolate into the egg yolks.

  Note: Did your chocolate "seize" up and turn slightly greasy? No worries, just vigorously whisk in a teaspoon of cold water at a time until it comes back together.
• Beat the egg whites: In a stand mixer (or with an electric hand mixer), beat 4 egg whites and 1 teaspoon Diamond-Crystal Kosher Salt until soft peaks form. Add 12 grams granulated sugar and continue beating until stiff peaks form.

  Stir in (don't fold) a quarter of the egg whites into the chocolate-egg mixture, then gently fold in the remaining egg whites until just barely incorporate. A few streaks of egg whites should remain so as not to overbeat and deflate the mixture.
• Chill: Transfer the mousse into a large serving bowl, individual ramekins, or glass pudding cups. Cover with plastic wrap, and store in the fridge for at least 4 hours, but ideally overnight.
• Serve: To serve the mousse, top with a bit of Flaky sea salt or the Chantilly cream (optional, recipe below). Enjoy immediately!

For the Chantilly cream (optional):

• Just before serving, make the whipped cream.

  In a stand mixer (or an electric hand mixer), beat 238 grams heavy cream and 1 teaspoon vanilla paste until stiff peaks form. Top mousse as desired, and enjoy.