Preheat oven to 325 degrees. Line an 8x8 inch or 9x9 inch baking dish with parchment paper, set aside.
Using the double broiler method (glass bowl on top of a medium sized saucepan filled halfway with water at a simmer) combine chopped semi sweet chocolate and butter. Allow chocolate and butter to melt completely, stirring regularly to get the chocolate and butter to fully combine. Once chocolate mixture is completely melted and combined, stick chocolate in the fridge (keep it in the same bowl) for a few minutes to cool the chocolate.
In a separate bowl, combine cocoa powder, flour and salt. Whisk or sift ingredients together so there are no clumps of dry ingredients. Set aside.
Remove chocolate mixture from fridge. Add in sugar, mixing until thoroughly combined. Chocolate may be gritty from sugar. Add in eggs and vanilla extract, mixing completely. Slowly add in dry mixture and continue stirring. Batter should pull away from the sides of the bowl as it thickens up. Add in Nutella and fold it into the batter until fully combined. Be patient, this may take a few minutes.
Pour batter into the baking dish and bake in the oven for 40-45 minutes. Brownies are down when a tester inserted into the center comes out clean. Brownies may seem super soft but will set as they cool. Allow at least 20-30 minutes of cooling time before serving brownies. Brownies are extremely moist, thick & fudgy. When ready to serve lift up sides of the parchment paper and transfer to a cool surface to cut into squares. Optional; serve brownies with vanilla ice cream
Any leftovers (even though there won't be any) should be stored in an airtight container to maintain freshness.