Preheat the oven to 350F and line & spray an 8×8 baking pan with parchment paper, leaving an overhang to easily remove brownies after baked
In a large microwave-safe bowl, combine the milk chocolate chips, dark/unsweetened Belgian chocolate and butter. Heat on 50% power or the defrost setting for 1 minute then remove and stir. Continue heating in 15-30-second intervals until the mixture is completely melted & smooth. Be careful not to overheat.
1 cup milk chocolate callets or chips, 4 oz unsweetened or dark Belgian chocolate, 1 cup Salted butter at room temperature
Stir the Nutella into the melted chocolate mixture.
1/2 cup Nutella
Using a stand mixer with the paddle attachment, or electric mixer, mix the eggs, egg yolk and sugar on medium-high speed until fully combined & mixture has lightened in colour – about 3-5 minutes.
3 eggs, 1 egg yolk, 1 1/4 cups sugar
Add the vanilla extract and Frangelico, if using, and beat for 30 seconds more.
1 tsp pure vanilla extract, 1 tbsp Frangelico
Remove from mixer, and stir in the cooled chocolate with a rubber spatula. Next, gently fold in the flour and cocoa powder.
3/4 cup cocoa powder, 1/2 cup all-purpose flour
Pour the brownie batter into the prepared baking pan and bake for 35-40 minutes. You want to bake until a toothpick or tester comes out with moist crumbs still attached.
Let cool to room temperature.