Preheat your oven to 350°F (175°C) and prepare an 8x8-inch baking pan by greasing it or lining it with parchment paper.

In a medium saucepan over low heat, melt 1 cup of unsalted butter, then whisk in 1 cup of cocoa powder until smooth.

Mix in 2 cups of sugar, then add 4 eggs one at a time, mixing well after each. Add 1 teaspoon of vanilla extract.

Sift together 1 cup of all-purpose flour and 0.5 teaspoon of salt, then fold into the wet ingredients gently.

Pour the batter into the prepared baking pan and bake for 25 to 30 minutes, checking doneness with a toothpick.

Allow to cool in the pan for 15 minutes before transferring to a wire rack to cool completely. Cut into squares and serve.