• Preheat the oven to 350°F. Grease an 8x8 baking pan, and then line with parchment paper, making sure to leave some paper hanging over the edges, as this will help you easily pull the brownies out of the pan once baked. Lightly grease the parchment after lining the pan as well. Set aside.
• In a large microwave-safe bowl, add the chocolate chips, rough chopped chocolate, and butter. Microwave for 1 minute, and then stir to combine. Keep microwaving in 30 second intervals, stirring in between until the mixture is perfectly smooth. Let it cool for a few minutes and then add the Nutella, stirring until smooth.
• In a large bowl or the bowl of a stand mixer, add the eggs and sugar. Beat together on medium-high speed until homogenous and thick, ~2-3 minutes. Add the vanilla extract and mix. The chocolate/Nutella mix should have cooled a bit by now, slowly stir it in and mix together by hand with a rubber spatula.
• In a small bowl, whisk together the flour, cocoa powder, and salt. Slowly add the dry ingredients to the wet, and gently fold in until fully mixed.
• Pour the batter into your prepped baking pan. Bake 35-40 minutes, a tester should come out with moist crumbs, you do NOT want the tester to come out clean; these brownies need to be fudgy! Err on the side of under-baked, it will set while cooling. Once done, remove from the oven and allow to cool in the pan before removing and decorating.

Make the Buttercream

• In the bowl of a stand mixer, beat the butter alone on medium-high for 6-7 minutes, until light and creamy. Reduce the speed to low and add the salt and powdered sugar slowly, and continue until the sugar is fully mixed.
• Beat in the vanilla extract and heavy cream and mix until combined. Once mixed, turn the speed back to medium-high and mix for an additional 6-7 minutes, this will give the buttercream a beautifully light texture.
• If you don't quite like the consistency, add either milk (1 tsp at a time) or more powdered sugar to thin it out or make it thicker respectively.

Decorate

• In a small microwave safe bowl, add the 4 oz semi-sweet chocolate. Similar to making the brownies, microwave the chocolate for 1 minute and then stir, continuing to microwave for 30 seconds until fully melted and smooth. Let cool for a bit so it's not too runny while piping the frosting.
• Pipe the frosting onto the brownies, followed by a thin line of melted chocolate. Add mini M&Ms as Christmas lights, and enjoy!