Step 1

To make the chocolate mousse: Place the chopped chocolate in a large heatproof bowl. Position a fine mesh sieve over the top.

Step 2

In a medium saucepan off the heat, whisk the egg yolks. Add the sugar and salt and whisk until well combined. Gradually add 1/2 cup (113g) of the heavy cream and whisk until fully incorporated, being sure to reach the creases of the pan. Set over medium heat and cook, whisking constantly, until the mixture reaches 160°F when measured with a digital thermometer (it will be thick enough to coat the back of a metal spoon). Pour the mixture through the sieve into the bowl with the chocolate. Add the vanilla and espresso powder. Discard any of the solids that collect in the sieve.

Step 3

Gently whisk the chocolate mixture until smooth, then let it cool to room temperature, stirring occasionally to promote even cooling, 20 to 30 minutes.

Step 4

In the bowl of a stand mixer fitted with the whisk, beat the remaining 3/4 cups (170g) of the heavy cream until you reach stiff peaks (a peak should support itself without drooping over). Whisk 1/3 of the whipped cream into the chocolate mixture to lighten it, then add the remaining whipped cream and gently fold until no white streaks remain.

Step 5

Transfer the chocolate mousse to a large serving bowl or to individual ramekins, small glasses, or jars. Cover and chill the chocolate mousse until set, at least 6 hours.

Step 6

To make the topping: When ready to serve, whip the cream to soft peaks using your favorite method: beating with an electric hand mixer or stand mixer fitted with the whisk, shaking a Mason jar, or whipping by hand with a whisk. Serve the chocolate mousse with a dollop of whipped cream on top, garnished with a sprinkling of mini chocolate chips or shavings from a chocolate bar.

Step 7

Storage information: Store covered chocolate mousse, without any toppings, in the refrigerator for up to 4 days.