Line a 9 in. square baking tin with parchment paper with an extra ¼ - ½ inch on each side.
Melt the butter and unsweetened chocolate together over a pan of boiling water. Stir until smoother and combined. Leave to cool for 5 minutes.
Sift the flour, baking soda, and cocoa powder together.
Whisk the eggs and cold brew into the chocolate mixture.
Add the flour mixture and sugar. Stir well to combine.
Pour batter into prepared pan. Swirl the Nutella into the brownies and bake for about 25 minutes. They should be fudgey and a little undercooked when removed from the oven. Bake for 30 minutes for a cake-like texture.