• Pre-heat your oven to 370℉/180℃. Line an 8" Square Pan with greaseproof paper.
• In a small bowl, break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn't burn. Alternatively, set up a double boiler. Do this by placing a heatproof bowl over a small pot of simmering water. Add the chocolate to the heatproof bowl and stir until melted. 
• In a large mixing bowl, whisk together melted butter and both sugars until fully combined (3-5 minutes). Add in the eggs and whisk again. Finally, add the melted chocolate and whisk to combine.
• Fold in the flour and cocoa powder. Finally, add the chopped white chocolate (or chocolate chips) and fold together.
• Pour into the lined tin and bake 30 minutes. Remove and let cool before frosting.

For the Frosting

• Add the butter and a handful (6/7) blackberries to a large bowl. Beat with a whisk on high for a minute to combine everything.
• Add half the powdered sugar then whisk on high. Then add the remaining half and whisk everything on high until fluffy. Add powdered sugar as required to meet your desired consistency (or use milk to thin). Spread over the cooled brownies, top with more blackberries - slice and serve!