Preheat oven to 350 F
Spray an 8x8 baking dish with non-stick cooking spray
In a large microwave safe bowl, combine margarine, brown sugar, stevia and cocoa powder and heat until butter is melted (takes 15-20 seconds)
Stir until all ingredients are well combined
Add in vanilla and eggs and stir
Pour in baking powder, flour, salt and blackberries and stir again
Pour batter into greased dish and bake for 35-40 minutes, or until completely done in center
Let cool completely before eating
Top with sweet glaze sauce (optional)
Enjoy!