Melt butter and chocolate in a medium heat-proof bowl over a saucepan of simmering water; remove from heat. Beat eggs, sugar, and salt with a hand mixer in a medium bowl until sugar dissolves. Beat egg mixture into chocolate until smooth. Beat in flour until just combined.
Adjust the oven rack to the middle position; preheat the oven to 450 degrees.
Butter ramekin dishes or if you don't have ramekins, line standard-size muffin cups (1/2 cup capacity) with 8 extra-large muffin papers (papers should extend above cups to facilitate removal). Spray muffin papers with cooking spray. Divide batter among ramekins/cups. Place ramekins on a silicon lined cookie sheet to bake.
Bake until batter puffs, but the center is not fully set, 8 to 10 minutes depending on the size of . Take the cakes out when still loose or "jiggly" in the center. (If using muffin tins, carefully lift cakes by the paper from the tin and set on a work surface. Pull papers away from cakes and transfer cakes to dessert plates.
Top each cake with powdered sugar or a dollop of fresh whipped cream and raspberries or strawberries. Serve immediately.
Note: Batter can be made ahead; return to room temperature an hour or so before baking.