Grease a 9-inch springform pan. If making a crust, bake it first and let cool. (This mousse can also stand alone).

Melt chocolate and butter together in a double boiler or microwave bursts, stirring until smooth. Let cool to tepid.

Whisk egg yolks into the cooled chocolate mixture one at a time until incorporated.

In a clean bowl, whip egg whites with salt until frothy. Gradually add sugar and whip to stiff, glossy peaks.

In another bowl, whip heavy cream and vanilla to soft peaks.

Gently fold egg whites into chocolate mixture in three batches. Then fold in the whipped cream just until combined. Do not overmix.

Pour into pan. Refrigerate for at least 6 hours or overnight before serving.