Heat oven to temperature on the brownie mix box and grease an 8x8 pan.
Prepare brownie batter per box directions; stir in melted butter if using and fold in mini chips.
Pour half the batter into the pan and smooth evenly.
Drop spoonfuls of Nutella across the batter, leaving small gaps between blobs.
Spoon remaining batter over the Nutella and gently spread to cover.
Bake 25–35 minutes, checking at the shorter end; center should be set but slightly soft.
Cool at least 15 minutes, then dust with powdered sugar or drizzle more Nutella if desired.