Heat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving a little overhang for lifting, or lightly grease the pan.

Whisk the melted butter, oil, and granulated sugar together in a large bowl until well combined. Add the eggs and vanilla extract, then whisk vigorously for about 1 minute until the mixture looks slightly lighter and glossy.

Sift or whisk the cocoa powder, flour, and salt together in a separate bowl. Add the dry mixture to the wet mixture and fold just until no dry streaks remain.

Fold in 1/2 to 1 cup of chocolate chips or chopped chocolate if desired. Spread the batter evenly in the prepared pan.

Bake for 20–25 minutes, until the edges are set and the center still looks slightly soft. A toothpick inserted into the center should come out with moist crumbs rather than wet batter.

Cool the brownies completely in the pan before slicing. For especially neat, dense squares, refrigerate them briefly before cutting.