Grease a 9-inch round or square baking pan with butter or cooking spray. Line the bottom with parchment paper to ensure the cake releases easily after baking. In a medium bowl, combine the flour, cocoa powder, baking powder, and salt.
Use a sifter to prevent lumps and to evenly distribute the ingredients. Set this mixture aside; you’ll add it to the wet ingredients later.
In a large mixing bowl, combine the melted butter, vegetable oil, granulated sugar, and brown sugar. Whisk them together until smooth and glossy. The combination of butter and oil gives the cake a moist, tender texture while keeping the edges chewy like a brownie.
Crack the eggs into the bowl, one at a time, whisking thoroughly after each addition. This helps incorporate air, which will give the cake its light structure. Stir in the vanilla extract for a warm, aromatic flavour.
Gradually add the dry ingredients to the wet mixture in three batches, folding gently with a spatula. Be careful not to overmix—stop as soon as the batter is uniform and no streaks of flour remain. Overmixing can make the cake dense and tough.
Gently fold in the chocolate chips or chunks, spreading them evenly throughout the batter. These will melt slightly during baking, creating gooey pockets of chocolate.
Pour the batter into your prepared pan. Use a spatula to spread it evenly and smooth the surface. The batter will be thick, so take your time to ensure an even layer for consistent baking. Place the pan on the centre rack of your preheated oven.
