Line an 8-inch square pan with parchment paper or foil and let it overhang on two sides. Spray lightly with cooking spray.
In a large bowl whisk the sugar and cocoa powder together. Set aside.
Add the butter and the milk to a medium microwave-safe bowl. Microwave on high for 45 seconds and stir to melt the butter. If the butter doesn't completely melt microwave in 15 second increments and stir well in between until completely melted. Stir in the vanilla.
Pour over the sugar and mix until well combined. Spoon into the parchment lined pan and evenly spread it out. Cover with plastic and refrigerate 1 hour or until firm.