Preheat your oven to 350F. Grease and line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
In a large bowl add in the melted butter, oil, brown sugar, granulated sugar, eggs, and vanilla extract. Mix until combined.
In a separate bowl, sift together the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix. Fold in the chopped walnuts then pour into the prepared pan.
Bake in the preheated oven for 35-40 minutes or until a toothpick inserted into the center comes out with moist crumbs (not wet batter). The top should have a crinkly appearance, and the edges should be set while the center remains slightly fudgy.
Allow the brownies to cool completely in the pan on a wire rack.
Once cooled, lift the brownies out of the pan using the parchment paper overhang. Cut into squares and serve!