Preheat your oven to 350°F (175°C). Grease or line an 8x8-inch baking pan with parchment paper.
In a medium mixing bowl, whisk together melted butter, oil, and sugar until well combined and glossy. Add eggs and vanilla, whisking for about a minute until the mixture is light and smooth.
Sift in the cocoa powder, flour, salt, and baking powder if using. Gently fold the mixture with a spatula until just combined—do not overmix.
Fold in the chocolate chips or chunks for added richness and texture.
Pour the batter into the prepared pan and smooth the top. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Let brownies cool completely in the pan before slicing into squares for clean edges and the fudgiest texture.