Preheat the oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper, then lightly coat with non-stick baking spray. Set aside.

In a large mixing bowl, sift together the flour, baking powder, salt, cocoa powder, and espresso powder. Set the dry mixture aside.

In a medium saucepan, combine the butter, oil, and ⅓ cup of the granulated sugar. Heat over medium, stirring often, until the butter is fully melted. Remove from heat and let it cool slightly.

In another large bowl, whisk together the eggs, egg yolk, vanilla extract (if using), and the remaining sugar until fully combined—about 30 seconds.

Gradually add the warm butter mixture into the egg mixture, pouring slowly and whisking continuously until everything is smoothly incorporated.

Add the sifted dry ingredients and chocolate chips into the bowl, and gently fold with a rubber spatula until just combined. Be careful not to overmix—stop as soon as no streaks of dry flour remain to keep the brownies chewy, not cakey.

Pour the batter into the prepared baking pan and smooth the surface evenly.

Bake for 28 to 30 minutes, or until the edges are set and the top appears glossy with slight cracks.

Transfer the pan to a wire rack and allow the brownies to cool completely before slicing.