1. Preheat oven to 180°C (360°F). Line a 20cm (8 in) square tin with baking paper. Place caster sugar and eggs in mixing bowl and whip 1 min/speed 4. Transfer into a bowl and set aside. Scrape mixing bowl out well but there’s no need to wash it.
2. Place chocolate and butter in mixing bowl and melt 4 min/60°C (140°F)/speed 1.
3. Add vanilla and flour. Mix 10 sec/speed 2.
4. Return egg mixture to mixing bowl and combine 20 sec/speed 3.5. Ensure mixture is fully combined. If necessary, scrape down and mix a further 10 sec/speed 3.
5. Transfer into the tin, scatter over the nuts and bake for 30 minutes, or until top is deep brown and just starting to crack. Brownie should still feel slightly soft to the touch. Allow to cool in the tin for 10 minutes, then transfer to a wire rack to continue cooling.
Cut into pieces with a sharp knife. Store in an airtight container for up to four days if not eating immediately (who are we kidding!).