Preheat oven to 300 degrees F. Spray an 8 by 8-inch baking dish with non-stick cooking spray and set aside.

Place butter in a large microwave-safe bowl. Heat in microwave until just melted. Stir in sugar and salt, and let cool slightly if warm.

Add in the eggs and vanilla. Break the yolks and stir with a wooden spoon just until blended.

Add in the flour, cocoa powder, and chocolate chips. Fold and stir the mixture with a wooden spoon just until combined.

Pour the batter into the prepared baking dish and spread into an even layer. Bake in the preheated oven just until barely set, about 40-55 minutes. A toothpick inserted into the center may have a hint of sticky batter; be careful not to over-bake.

Let cool completely on a wire rack before cutting into squares. Store in an airtight container at room temperature.