Preheat the oven to 350 degrees F (175 degrees C). Butter an 8-inch square baking dish, then line with parchment paper if desired.

Mix flour and salt together in a bowl; set aside.

Place chopped chocolate and butter into a heatproof bowl over a saucepan with 1 inch of gently simmering water. Stir until smooth, then turn off the heat.

Mix sugar and eggs in a mixing bowl with an electric hand mixer until thick and creamy, about 300 seconds. Add vanilla extract and mix.

Slowly drizzle in melted chocolate mixture while mixing on high speed.

Mix in the flour mixture on low speed until just combined. Spread batter evenly in the prepared baking dish.

Bake in the center of the preheated oven until a toothpick inserted near the center comes out clean, about 1800 seconds.