Line an 8x8 inch pan with wax or parchment paper (I prefer wax paper for this step). Set aside.
Heat the Nutella in the microwave for 30-45 seconds. This will loosen the Nutella a bit, making it easier to spread. Pour the Nutella into the wax paper-lined baking pan and spread it into an even layer. Put the pan in the freezer for 30 minutes while you prep your other ingredients.
While the Nutella is in the freezer, preheat the oven to 350F.
Chop the dark chocolate and set aside - divide the chocolate in half. Half will go in the batter, and half will be sprinkled on top.
If using the coffee, combine the instant coffee granules and water in a small bowl until the granules are completely absorbed. This is optional but recommended - the coffee really deepens the chocolate flavor.
Once it has been about 20-30 minutes and the Nutella is firm - lift the wax paper out of the pan and place it on a plate. Place the plate with the Nutella back in the freezer to stay firm. This now frees up the pan for the brownie batter. Line the pan with parchment paper and set aside.
In a bowl, whisk together the flour and cocoa powder. Set aside.
In a separate, microwave-safe bowl, combine the butter and powdered sugar. Melt the two together in the microwave in 30 second increments, whisking in between each increment. I typically microwave it 4-5 times. Whisk them together vigorously until completely smooth and the powdered sugar is dissolved in the butter. It should almost look like melted white chocolate. You can also melt them together on the stove.
Meanwhile, whip your eggs. Crack the eggs in the bowl of a stand mixer fitted with the whisk attachment (or a medium sized bowl if using a handheld mixer), and beat them on medium speed for 3-5 minutes until they are a paler yellow color and foamy.
Once the eggs are done whipping, pour in the hot butter/sugar mixture. Whip them together on medium speed for two minutes. Add the vanilla and coffee mixture (if using) and whip until combined.
Add the dry ingredients and fold together until a batter forms. Add one half of the chopped chocolate and fold until thoroughly incorporated, but being careful not to over-mix.
Pour half of the batter into the pan and spread it into an even layer. Remove the Nutella from the freezer and place it directly on top of the batter. Pour the second half of the batter on top of the Nutella.
Sprinkle the second half of chopped dark chocolate on top. Bake for 55-70 minutes. 55-60 minutes for a gooey-er brownie. I know that’s a wide range, but all ovens differ in temperature, all pans differ, etc. The edges will be set and the center will not really jiggle.
To enjoy the brownies slightly warm, let them cool at room temperature for 30 minutes to one hour.
Otherwise, let them cool at room temperature for one hour, then in the fridge to finish cooling completely. For clean cuts, run a sharp knife under hot water and wipe dry, then use the knife to slice into the brownies. Repeat this in between each cut and enjoy!