For chocolate pastry, process flour, icing sugar and cocoa in a food processor until combined. Add butter, process until mixture resembles fine crumbs, then add egg yolks, process to combine. Turn onto a lightly floured work surface and bring pastry together with the heel of your hand. Wrap in plastic wrap, refrigerate for 1 hour to rest.
Preheat oven to 180C. Roll out pastry on a lightly floured surface to 3mm thick and linea 28cm-diameter loose-bottomed tart tin, trimming edges. Refrigerate for 1 hour, then blind bake for 8-10 minutes, remove paper and weights and bake until dry and crisp (8-10 minutes).
Meanwhile, for praline filling, spread hazelnuts on an oiled baking tray, set aside. Combine sugar and 60ml water in a small saucepan, stir over medium-high heat until sugar dissolves. Bring to the boil, cook until dark caramel in colour (4-5 minutes), pour over nuts. Stand until cool and set (8-10 minutes), process in a food processor until finely ground, set aside. Bring cream to the simmer in a small saucepan over medium heat, add chocolate, stir until smooth, remove from heat, stir in two-thirds of praline mixture (reserve remaining to serve). Spoon into pastry case, smooth top, refrigerate until just set (1½-2 hours).
Combine cream and milk in a small saucepan, bring to the simmer over medium-high heat. Add dark chocolate, remove from heat, stir until smooth. Spread over tart, refrigerate until just set (45 minutes-1 hour). Cut into wedges with a hot knife and serve immediately scattered with reserved praline.