Preheat your oven to 350°F (175°C) and grease a 9×13-inch sheet pan or line it with parchment paper.

1. In a medium saucepan, melt the butter and chocolate together over low heat, stirring constantly until smooth and well combined.
2. Remove from heat and whisk in the cocoa powder, sugar, salt, and vanilla extract until fully combined.
3. Add the eggs one at a time, beating well after each addition. The batter will be thick and shiny.
4. Pour the batter into the prepared baking dish and spread it evenly with a spatula.
5. Bake for 20-25 minutes, or until the edges are set and the center is slightly puffed. A toothpick inserted into the center should come out with a few moist crumbs.
6. Allow the cake to cool completely in the pan before slicing into squares or rectangles.
7. Serve as-is or top with whipped cream, berries, or a dusting of powdered sugar.