Put milk into a medium saucepan and heat over medium heat until just hot, not boiling.
Roughly combine egg yolks, sugar, corn flour and cocoa in a large bowl using a whisk. Add ½ cup of hot milk and whisk until smooth. Add remaining milk, whisking until combined, then return mixture back to saucepan. Heat over a medium heat, stirring constantly with a whisk for about 4 minutes or until mixture is very thick and smooth. Remove from heat and stir in chocolate until combined. Transfer to a bowl and cover mixture directly with a sheet of baking paper to prevent a skin from forming. Refrigerate for 1 hour or until cold. (You can make this the day before if you prefer.)
Meanwhile, preheat oven to 190C fan-forced. Line three oven trays with baking paper. Cut pastry sheets in half to form four rectangles and arrange on two of the prepared trays. Cover each with a second sheet of baking paper, and a second oven tray to weigh the pastry down. Bake for 20 minutes, remove top trays and top sheet of baking paper and return pastry to the oven for 2-5 minutes or until golden. Set aside to cool completely.
Meanwhile, to make toffee, put extra sugar and 2 tablespoons of water into a small saucepan over medium heat. Cook stirring until sugar dissolves. Increase heat to high and cook without stirring until amber in colour. Immediately pour toffee onto remaining prepared tray, and spread into a thin layer. Set aside for 15 minutes or until cooled. Roughly chop.
Briefly whisk cooled custard mixture until smooth, then spoon into a piping bag fitted with a 1cm star nozzle. Pipe mixture down the length of three pastry rectangles to cover completely, then stack them on a serving platter, followed by the fourth piece of pastry.
Spoon cream on top. Arrange Ferrero Rocher on top and decorate each with a little gold leaf. Scatter with hazelnuts, a little toffee and chocolate curls. Slice into pieces using a serrated knife, serve.