Step 1

Preheat oven to 170°C/150°C fan-forced. Grease a 6cm-deep, 20cm (base) round springform pan. Line base and side with baking paper.

Step 2

Place butter and 200g chocolate in a large heatproof bowl. Pour water into a saucepan until one-third full. Bring to the boil over high heat. Reduce heat to low. Place bowl over saucepan (don’t let bowl touch water). Using a metal spoon, stir mixture for 3 minutes or until smooth and combined. Cool slightly.

Step 3

Combine sugar, flour, cocoa, 2 tablespoons walnuts and two-thirds remaining chocolate in a large bowl. Add butter mixture and eggs. Stir to combine. Pour into prepared pan. Sprinkle with remaining walnuts and chocolate. Bake for 1 hour or until a skewer inserted in centre comes out with crumbs clinging. Cool completely in pan.

Step 4

Dust with cocoa. Serve.