Preheat oven to 350°F (180°C).

Melt chocolate and butter in a double boiler, stirring occasionally, until smooth.

Whisk together sugar, eggs, and vanilla extract in a large bowl.

Combine the melted chocolate mixture with the sugar mixture, whisking until well incorporated.

Sift in flour and salt, then fold the mixture until just combined.

Pour the batter into a greased and floured 8-inch square baking pan.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.

Remove from the oven and let cool completely in the pan before cutting into squares.

Cut into squares and serve.