Preheat the oven to 350 degrees. Prepare a 13 x 9" baking pan with non-stick cooking spray and parchment paper.
In a medium bowl, whisk together flour and cocoa powder. Set aside.
In a mixer fitted with a paddle attachment, cream butter and sugar until light a fluffy.
Add eggs and vanilla. Mix on medium speed for about 5 minutes.
Add melted chocolate. Mix until combined.
Add flour and cocoa mixture. Mix until just combined. Don't OVERMIX!
Pour batter into a pan and use a offset spatula to make the batter even.
Bake for 35-40 minutes, or until a toothpick comes out clean.
Set on a wire rack to cool and then cut into squares.