Preheat oven to 375 degrees F.
Line the bottom of a spring form pan with parchment paper.
Spray the sides of the pan and the parchment paper with cooking spray.
Melt the chocolate and butter in saucepan over medium-low heat, stirring to fully combine.
Transfer chocolate butter mixture to a large bowl.
Add sugar and mix to combine.
Add eggs one at a time whisking thoroughly into mixture before adding the next egg. The mixture will become increasingly thicker.
Add cocoa powder in quarter cup increments until combined.
Pour mixture into prepared pan.
Bake for 35-40 minutes for a 9 inch round pan or 25-30 minutes for a 10 inch round pan or until cake is firm in the center.
Cool for about 10 minutes and then invert and turn onto a plate, removing the sides of the pan.
Discard parchment paper.
Top with powdered sugar, berries or ganache