Line the bottom of a 9-inch spring form pan with parchment paper.  Grease the top of the paper and the sides of the pan.  Set aside.

Melt chocolate and butter in a double boiler or in a metal bowl over simmering water, stirring frequently, until smooth.  Remove from heat.  Add vanilla extract and stir until smooth.  Set aside.

Whisk eggs and salt in the bowl of a standing electric mixer at high speed until they triple in volume and turn very pale yellow, about 8 minutes.   Fold half of the chocolate mixture into to egg mixture with a silicone spatula.  Sprinkle flour over the top of the egg-chocolate mixture and add the remaining chocolate.  Fold until fully incorporated.

Pour mixture into prepared pan.  Bake for 20 minutes in an oven preheated to 375° F.  The center will still be soft.  Remove from the oven and cool for at least 30 minutes before cutting.

To prepare ahead of time, after removing the cake from the oven, cool to room temperature, top the pan with plastic wrap, and refrigerate for up to 2 days.  Let sit at room temperature for about an hour to take the chill of, if desired.