In a mixing bowl, whisk together the egg yolks and 25g of granulated sugar until pale yellow and slightly thickened.

In a medium saucepan, combine heavy cream, strongly brewed coffee, vanilla extract, and salt. Heat over medium heat until edges bubble, about 5-7 minutes.

Gradually pour the hot cream mixture into the egg yolks while whisking continuously. Return mixture to saucepan and stir on low heat for about 5 minutes until thickened.

Remove from heat and let the custard cool to room temperature.

In a clean mixing bowl, beat egg whites with cream of tartar and remaining sugar until stiff peaks form, about 4-6 minutes.

Gently fold meringue into the cooled custard until no streaks remain.

Spoon the mousse into cups, cover with plastic wrap, and chill in the refrigerator for 1-2 hours.

Before serving, dust with cocoa powder and garnish with coffee beans.