Grease a 20cm/8" square pan with butter then line with parchment/baking paper with overhang.

Scrape Nutella into pan, spread. Freeze 1 hour or until solid.

Using paper overhang, remove onto a plate and return to freezer (softens pretty quickly)

Preheat oven to 180C/350F (standard) or 160C/320F (fan forced / convection).

Reline the same pan with parchment/baking paper with overhang.

Place butter and chocolate chips in a heatproof bowl, microwave in 30 second bursts (takes me 1 min) until almost melted. Stir to finish melting and incorporate butter into chocolate.

Cool for a few minutes. Add eggs, sugar and vanilla, whisk to combine.

Add Dry ingredients, whisk to combine (few lumps ok).

Pour half batter into pan. Use spatula/egg flip to lift frozen Nutella slab and place on top. Top with remaining batter.

Bake 35 minutes. Insert skewer in middle to check if cooked - should come out with mostly pale brown smear (Nutella) and a bit of dark brown smear (brownie, fudgy).

Cool in pan 30 minutes, then use paper overhang to lift out. Cool for at least another 45 minutes then slice into 16 pieces (4 x 4) and serve!