Method

• 1.
  Place hazelnuts and flour in a food processor and whiz until hazelnuts are very finely chopped. Add caster sugar and ¼ tsp fine salt and whiz until combined. Add butter and whiz until combined. Add water and whiz until dough forms a ball. On baking paper, roll dough into a rough 35cm circle. Transfer to a large tray and chill for 1 hour to rest.
• 2.
  Stand dough at room temperature for 15-20 minutes, until just pliable. Line base and side of a 26.5cm x 3.5cm deep loose-bottomed fluted tart pan with dough circle, leaving a 1cm overhang (see notes). Reserve dough scraps. Freeze for 1 hour to rest.
• 3.
  Preheat oven to 200°C/180°C fan- forced and place a baking tray in the oven to preheat. Line pastry with foil and fill with baking weights or dried beans.
• 4.
  Bake pastry for 50-55 minutes, until evenly deep brown. Remove foil and weights. Stand on a wire rack and cool slightly then, using a serrated knife, carefully cut off excess pastry. Patch any tears in pastry with reserved dough scraps.
• 5.
  Meanwhile, for butterscotch, combine thickened cream, butter, coffee and 1 tsp salt flakes in a small bowl. Cook caster sugar in a medium saucepan over medium-high heat, swirling pan occasionally, for 6-8 minutes, until a dark amber caramel. Working quickly, remove pan from heat and, being careful of the splatter, stir in cream mixture (it will partially seize). Set aside to cool slightly.
• 6.
  Pour butterscotch over tart base. Cool both layers to room temperature. Reduce oven to 150°C/130°C fan-forced.
• 7.
  For the tart topping, in a stand mixer fitted with the whisk attachment, whisk egg, egg yolks and icing sugar at medium-high speed for 10 minutes, or until pale and fluffy. Meanwhile, melt chocolates and double cream in a large heatproof bowl over a saucepan of simmering water (don’t let base of bowl touch the water), stirring occasionally, until smooth, thick and glossy. Don’t overheat or mixture will split. Fold one-third of egg mixture into chocolate mixture. Fold in remaining egg mixture with 1 tsp salt flakes until just combined.
• 8.
  Pour chocolate mixture on top of butterscotch, smooth surface lightly and bake for 15 minutes. The top will form the lightest crust, especially at the edges, and the centre will still be a little shiny. It should feel firm and squidgy-soft to touch (see recipe notes). Set aside to cool completely.
• 9.
  Serve dusted with cocoa powder.