Place the chocolate and butter in the top of a double boiler and melt until smooth. Cool slightly, until just warm to the touch.
Preheat the oven to 350 degrees F. Butter the inside of a 9-inch springform pan.
Place the egg whites in a medium bowl with the salt. Place the yolks in a slightly larger bowl with the granulated sugar. Using a handheld mixer, beat the whites on medium speed until stiff peaks form, about 2 minutes. Then beat the egg yolks on medium speed until light, pale and fluffy, about 1 1/2 minutes. Using a rubber spatula, stir the chocolate mixture, almond flour and vanilla into the yolks. In 2 batches, fold the whipped whites into the chocolate base. Pour the mixture into the prepared pan and bake until puffed and beginning to crack on top, about 40 minutes. Cool completely on a wire rack.
When cool, run a thin knife around the edge of the pan to release the cake. Remove the outside ring and dust the top with confectioners' sugar.