Preheat the oven to 350 °F (175 °C) and line an 8"×8" (20cmx20cm) pan with parchment paper.
In a medium mixing bowl, stir together the flour, cocoa powder, and salt. Set aside.
1 cup (125 grams) flour, 3/4 cup (86 grams) Dutch-process cocoa powder, 1/4 teaspoon salt
In a large mixing bowl, stir together the butter or oil, sugar, and vanilla extract. Once combined, add the eggs one at a time, and stir just until combined.
3/4 cup + 2 tablespoons (196 grams) unsalted butter, 1 1/2 cups (300 grams) granulated sugar, 1 tablespoon vanilla extract, 3 large eggs
Add the dry mixture to the wet and stir just until almost no streaks of flour remain. Do not overmix!
Pour the batter into the prepared pan.
Bake for 15-20 minutes or until the brownies have formed a thin crust and appear set in the middle. A toothpick inserted into the center will come out wet. A toothpick inserted into the sides will come out with some moist crumbs on it, but not totally raw batter. The brownies will continue to bake as they sit in the pan and will firm up as they cool.
Let cool completely, about 2 hours, and then cover and store at room temperature for up to 3 days. You can also refrigerate them to give them a fudgier texture.