Cut out an 8x8-inch square piece of parchment paper. Dollop 1/2 cup (140g) Nutella onto the paper, and spread out to the edges. Freeze until solid - about 1 hour.

Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper, and set aside.

Place the remaining Nutella, butter, sugar, cocoa powder, and salt into a large microwave-safe bowl, and heat for 1 minute. Stir well, then microwave for another minute. The mixture should very warm to the touch.

Add the eggs and vanilla, and mix well until combined. Fold in the flour until fully combined, then gently fold in the chocolate chips. Be sure not to let them ALL melt into the warm batter.

Spoon half of the batter into the prepared pan and spread out evenly. Place the frozen Nutella on top, then top with the rest of the brownie batter.

Bake for 45 - 50 minutes, or until the centre is no longer jiggly. Allow to cool completely in the pan on a wire rack before cutting into bars.