Preheat the oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two opposite sides so the brownies can be lifted out easily. You can also lightly grease the pan if you don't have parchment.

Melt the butter in a medium saucepan over low heat. When it is about 70 percent melted, add the cocoa powder and whisk until the mixture is smooth. Remove the saucepan from the heat.

In a separate bowl, beat the eggs and sugar together for at least 4 minutes, until the mixture is pale and has doubled in volume.

Once the butter and cocoa mixture has cooled slightly, whisk in the vegetable oil and vanilla extract.

Beat the egg mixture into the melted butter and cocoa mixture until combined.

Using a rubber spatula or wooden spoon, gently fold in the flour and salt until just incorporated.

If you like, stir in the chocolate chips and chopped walnuts.

Pour the batter into the prepared pan and bake for 28 to 32 minutes, until the top looks set and a toothpick inserted in the center comes out with a few moist crumbs.

Cool the brownies completely in the pan. When the pan no longer feels warm, lift the whole batch out using the parchment overhang, transfer to a cutting board, and slice into squares.