Prepare a double boiler by placing two medium heatproof bowls over a lightly simmering pot of water.

In one bowl, whisk together the egg yolks and caster sugar until foamy, heating for about 3–8 minutes until thickened. Cool quickly in ice water.

In another bowl, melt the chopped chocolate and espresso powder with the hot water over simmering water, stirring until smooth. Remove from heat.

Mix the melted chocolate into the cooled egg yolk mixture.

Beat egg whites and kosher salt in a clean bowl until soft peaks form. Gradually add granulated sugar, continuing to beat until stiff peaks form.

Fold a quarter of the egg whites into the chocolate mixture to lighten it, then gently fold in the remaining egg whites in two additions.

Transfer mousse to serving dishes, cover, and chill in the fridge for at least 4 hours or overnight.

Before serving, whip heavy cream and vanilla paste until stiff peaks form, and top the mousse with the whipped cream.