• Preheat the oven to 350°F. Line an 8x8 baking pan with foil, parchment paper, or generously grease the pan with baking spray, set aside.
• In the body of a stand mixer or in a large bowl with a hand mixer, add the butter and mix until smooth.
• Add in the brown sugar and mix to combine well, scrape down the sides as needed. Add in the eggs one at a time until fully incorporated.
• Add in the Nutella and salt, mix until fluffy, about 2 minutes, scrape down the sides as needed.
• Fold in the flour until there are no dry patches, do not over-mix.
• Pour batter into the prepared baking dish and bake for 32-34 minutes until a toothpick inserted into the center comes out clean. Place on a wire rack to cool completely in the pan.

Buttercream

• In the body of a stand mixer or with a hand mixer, whip the butter until smooth. Add in the powdered sugar, mix it in on low speed until combined, scrape down the sides as needed.
• Add in the vanilla and heavy cream, mix in, then whip on high for 3 minutes until fluffy, scraping down the sides as needed.
• Once the brownies are completely cool, you can either take them out of the pan and frost them with the buttercream, or frost them in the pan.
• Take the chocolate bar and, using a handheld peeler, make chocolate shavings for garnish on the top.