Preheat the oven to 350 degrees, and prepare 2 8x8-inch square baking pans with parchment paper and nonstick spray.
In a heatproof bowl over simmering water, heat dark chocolate and butter until nearly fully melted. Remove from the heat and stir until smooth. Stir in sugar and eggs one at a time. Add salt and then flour until just mixed.
Divide batter between prepared baking pans and spread evenly. Bake for 15 minutes or until the center is set. Cool completely.