• Pre-heat your oven to 180℃ Fan/370℉. 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, or the bowl of a stand mixer, whisk together melted butter and both sugars. Then add the melted chocolate and combine. Add the eggs and vanilla and whisk until the smooth batter comes together.
• Add the flour and cocoa powder. Fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Pour into the lined tin. 
• Bake 25-30 minutes until the top is shiny and it's still just a little wobbly in the middle. Let cool.

Make the Frosting

• Add the butter to a large mixing bowl. Beat with an electric whisk on high for a minute to soften.
• Add the vanilla paste and half the powdered sugar and whisk. Then add the remaining powdered sugar and the heavy cream/milk until it forms a smooth buttercream. Add more milk/cream if needed to reach a spreadable consistency.
• Spread the frosting on the cooled brownies, slice and serve! Tip: For clean slices, use a sharp knife and wipe clean after each cut.