Preheat the oven to 350F.

Grease a 13x9 inch baking pan with butter or nonstick cooking spray.

In a medium saucepan, melt butter and unsweetened chocolate over low heat, stirring until smooth.

Remove from heat and stir in cocoa powder, sugar, and salt until well combined.

Blend in the vanilla extract.

Beat in the eggs one at a time, stirring quickly to prevent cooking.

Blend in the all-purpose flour until just combined.

Scrape the batter into the prepared baking pan and smooth the top with a spatula.

Bake for 25-30 minutes, or until the edges begin to pull away from the pan and a toothpick inserted into the center comes out clean.

Let the brownies cool completely in the pan on a wire rack.

Once cool, cut into 12 or 24 squares and serve.