Preheat oven to 325°. Line a 9x9 baking pan with parchment paper, set aside.
Place butter, cocoa and sugar in a large microwave-safe bowl and microwave for about 1 minute and 30 seconds, or until butter is melted. Mix everything together until smooth. Add in eggs and vanilla and mix until smooth. Add chocolate chunks at this point because it is hard to evenly distribute when flour is added. Pour in the flour and mix until just combined. Then, mix some more until smooth and somewhat glossy. The batter will be very thick.
Pour the batter into your prepared baking pan and smooth it out until level. Bake for 25-30 minutes or until a toothpick comes out slightly moist with a little bit of batter left on it - this is what makes them fudgey. Let them cool in pan for 10 minutes then pull out using the parchment paper and set on wire rack to finish cooling completely.
Brownies will stay fresh for up to 5 days stored in an airtight container