Melt the butter in a second microwave safe bowl and leave to cool for about 2 minutes. Add the eggs and sugar and whisk well.
Add the flour and cocoa powder to the bold and mix to incorporate everything.
Fold in the chocolate chunks. *Chocolate chips can be substituted for the chocolate chunks.
Using a cookie scoop, ice cream scoop or a large spoon, drop spoonfuls of the brownie and blondie batters in an alternating pattern. Continue with this until the pan is full and all the mix has been used.
There may be some gaps between the mixes. Don't worry about that. They get filled in when the mixes get swirled together.
Use a toothpick, skewer or a knife with a thin blade and swirl the brownie and blondie mixes together.
Bake in the preheated oven for 30-35 minutes. Once baked, remove from the oven and place the pan on a cooling rack to cool completely before slicing.