Step 1

Preheat the oven to 350 degrees F. Generously butter a 13-by-9-inch baking pan or spray it with nonstick cooking spray.

Step 2

Place the butter and chocolate in a medium-size saucepan over low heat and let melt together, stirring until smooth. Remove the saucepan from the heat and stir in the cocoa powder, sugar, and salt, then blend in the vanilla. Beat in the eggs one at a time, stirring to mix quickly so they don't have a chance to cook at all before they are blended in. Blend in the flour.

Step 3

Scrape the thick batter into the prepared baking pan and smooth the top with a spatula. Bake until the edges just begin to pull away from the sides of the pan and a wooden skewer or toothpick inserted into the middle comes out clean, 25 to 30 minutes.

Step 4

Let the brownies cool in the pan on a wire rack. When completely cool, cut them into 12 or 24 squares.