• Pre-heat your oven to 180℃ Fan/390℉. Line a 12 hole muffin tin with muffin cases.
• 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. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs, one at a time and mixing in between. 
• Now, add the melted chocolate slowly and combine once again. 
• Add the flour, salt and cocoa powder, fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Scoop into the muffin tin holes, no need to worry about filling close to the top, they don't rise much at all.
• Bake 20-25 minutes, until no longer wobbly. While still warm, create the indents by using a teaspoon measure, egg cup or end of a wooden spoon. Press into the middle, leaving tall sides. Let cool.

For the Filling

• Add the butter and Nutella to a large mixing bowl. Beat with a whisk on high for a minute to soften/combine everything.
• Add half the powdered sugar, whisk on high. Then add the remaining half and whisk everything on high until fluffy. Add powdered sugar/milk as required to meet your desired consistency. Pipe into the brownie cups and top with some mini Kinder Buenos if using. Enjoy!