Preheat the oven to 350 degrees (F).
Grease and flour a 9-inch cake pan. Line the bottom of the pan with parchment paper.
In a microwave-safe bowl, melt the butter and chocolate in the microwave at half power in 30-second increments, stirring when the mixture begins to melt. Stir until smooth. Set aside to cool.
Beat the eggs and sugar until thick and light yellow. Add the vanilla and continue beating. Add the 1/3 cup flour and 1/4 teaspoon salt and mix. Add the chocolate mixture and mix until combined.
Pour into the pan, sprinkle with 2 generous pinches of flaked salt, and bake 20 - 25 minutes, or until the center is just set.
Cool completely in the pan before turning onto a serving plate and peeling off the parchment paper.
Serve plain or with Sweetened Whipped Cream and summer berries.
Beat the cream until it begins to take a shape. Add the powdered sugar and vanilla. Beat until it holds a peak.