Place the butter, icing sugar and flour in a food processor and pulse a few times until it looks like wet sand. Add the water 1 tablespoon at a time and continue to just pulse between each addition until the mixture just starts clumping together.
Turn the dough out onto a clean work surface and just pull it together with your hands into a flat disk. Wrap in plastic wrap and refrigerate for ½ an hour.
Roll the chilled dough out on a lightly floured surface to about 3-4 mm thick. Carefully lift the pastry into a non stick 8 inch round fluted tart tin with removeable base. Nudge and work the pastry into the corners and up the sides. Use kitchen scissors to trim off all but 1 cm of pastry dough above the top edge of the tin. Dock the base (poke holes all over with a fork) then chill in the freezer for 15 mins.
Preheat the oven to 180C / 350F / 160C fan forced. Place the tart tin on top of a baking tray (this makes it easier to move in and out of the oven). Line the tart shell with baking paper and fill with pie weights.
Bake for 20 minutes before removing the pie weights and baking paper. Bake for a further 10 minutes, then remove from the oven and use a sharp knife to run along the top edge of the tin to cut off the excess. Return it to the oven and bake for a final 10-15 minutes until golden all over. Brush the base with the egg white and bake for a further 2 minutes.
For the caramel (have all your ingredients ready before you begin)
Place the sugar and water in a heavy based saucepan over low heat and stir it gently with a metal spoon as the sugar starts to clump. You don't need to stir constantly but you need to be close by and stirring a few times each minute and use the back of the spoon to break up any very large clumps. This is very important so the sugar doesn't burn and dissolves evenly. It will take around 20 minutes.
Once the sugar is all dissolved, turn the heat up to medium and let it come to a simmer. The caramel should be amber gold in colour.
Work quickly but carefully as it can darken very quickly now. Add the butter. Whisk it in quickly until fully combined. Now add the cream and salt (be careful as it may splatter and bubble up) and do the same until all mixed through. By now the caramel should be a deep golden brown colour - if not allow it simmer, stirring regularly, until it is. Allow it to cool for 5-10 minutes.
Scatter the macadamias over the tart base, then pour the caramel all over. Use a spatula to nudge the caramel and macadamias around so you have an even spread of both. Refrigerate for 2 hours.
For the caramelised white chocolate ganache
Break up the caramelised white chocolate into small chunks and set aside in a heatproof bowl. Heat the cream in a small saucepan and as soon as bubble start to appear at the edge, pour it over the chocolate. Let it sit for 3 minutes before stirring to a smooth ganache, then leave to firm up in the fridge for about 1 hour.
Spread the ganache over the tart, sprinkle with some extra sea salt flakes and serve.