Preheat the oven to 350°F (175°C). Line the baking pan with parchment, leaving an overhang for easy removal. Lightly grease the paper.
In a small saucepan or microwave-safe bowl, melt the butter until just melted—not sizzling. Whisk in the oil to combine.
In a medium bowl, whisk the granulated sugar, brown sugar, eggs, egg yolk, and vanilla for about 1–2 minutes, until thickened and slightly glossy. This step helps achieve a shiny top.
Whisk the melted butter mixture into the eggs until smooth. Sift in the black cocoa and natural cocoa powder; stir until no dry streaks remain.
In a separate small bowl, whisk together flour, baking powder, salt, cinnamon, cayenne, and cloves (if using).
Gently fold the dry mixture into the wet batter until just combined. Fold in chocolate chips or chunks. Avoid overmixing.
Pour batter into the prepared pan, smoothing the top. Sprinkle with turbinado sugar and a pinch of flaky salt.
Bake for 24–28 minutes, or until a toothpick inserted near the center has moist crumbs but no wet batter.
Cool completely in the pan on a wire rack before slicing into 9–12 squares. For cleaner slices, chill in the fridge for 30 minutes first.
Optional: dust lightly with cocoa powder and cinnamon before serving.