Preheat oven to moderately slow, 160°C, and lightly grease a 23cm springform flan pan.
Place flour, butter, nuts and icing sugar in a food processor and process until well-combined, add egg yolks and water and process until the ingredients just come together.
Turn onto a lightly floured surface, knead gently, wrap in plastic wrap and chill for 30 minutes.
Roll out the pastry between 2 sheets of baking paper, ease into prepared pan, trim the edges and chill for 10 minutes.
Bake blind (see Note below) for 15 minutes, remove paper and filling, and bake for another 10 minutes.
FILLING: Melt 250g chocolate in a small saucepan over a low heat; in a small bowl, using an electric beater, beat the egg yolks, eggs and caster sugar until the mixture is thick and creamy; stir in the cream, vanilla extract and melted chocolate.
Sprinkle the pastry base with macadamia nuts, pour the chocolate mixture into the pastry case; bake for 35-40 minutes or until the filling is set.
Cool to room temperature.
Serve the tart, drizzled with the remaining melted white chocolate.
NOTE: To blind bake, put rice, dried beans or pastry weights in the pastry shell, then bake as directed.