Preheat your oven to 350°F (175°C) and prepare your baking pan with parchment paper and non-stick spray.

Sift together the flour, salt, and cocoa powder in a medium bowl; set aside.

Melt butter and oil together in a saucepan and mix with sugar over medium heat.

Whisk together eggs and sugar in a large bowl, then add vanilla extract.

Combine the cooled butter and oil mixture into the egg mixture while whisking.

Gently fold the dry ingredients into the wet mixture until just combined.

Fold in chocolate chips and any custom add-ins.

Pour the batter into the prepared pan, smoothing the top.

Bake for 30-40 minutes, until edges are firm and the center is slightly soft.

Remove from oven and cool on a wire rack for about 2 hours before cutting.