01 -
Preheat oven to 175 degrees C (350 degrees F). Line a 9x9-inch baking pan with parchment paper and lightly spray with non-stick baking spray. Set aside.
02 -
In a large bowl, sift together the flour, baking powder, salt, cocoa powder, and espresso powder. Set aside.
03 -
In a medium saucepan, combine the butter, oil, and 1/3 cup of the sugar. Heat over medium, stirring frequently, until the butter melts completely. Remove from heat.
04 -
In a large mixing bowl, whisk together the eggs, egg yolk, vanilla (if using), and remaining sugar for about 30 seconds until well combined.
05 -
Slowly pour the warm butter mixture into the egg mixture, adding it gradually while whisking constantly until fully combined.
06 -
Gently fold the dry ingredients and chocolate chips into the wet mixture using a rubber spatula. Stop mixing when the last traces of dry ingredients disappear. Avoid overmixing to prevent cakey brownies.
07 -
Pour the batter into the prepared baking pan and smooth the top evenly.
08 -
Bake for 28-30 minutes, or until the edges are firm and the top is shiny and slightly cracked.
09 -
Place the pan on a cooling rack and cool completely before slicing. Adjust bake time as needed for desired gooeyness or structure.