Mix the flour and icing sugar in a large bowl, add cubed butter and work it in the mixture until it resembles coarse crumbs. Add the egg and a bit of milk just to bring the dough together, form it into a ball.  Wrap it in a plastic wrap and put it in the fridge for 30 min.

Sprinkle your work surface and the rolling pin with some flour, take the dough out of the fridge and roll it out big enough to fit a 9" loose bottomed tart tin.

Grease the tart tin with some butter, drape the dough over the rolling pin and transfer it into the tart tin making sure it comes up the sides. Trim off the extra bits and prick it with a fork all over, cover it with a plastic wrap and put it back in the fridge for 30 minutes.

Pre-heat the oven to 180C/350F. Take the tart dough out of the fridge and line it with parchment paper pushing it into the sides, fill the tart with baking beans or rice and bake it "blind" for 10 minutes, take it out, removes the baking beans and parchment paper and bake for 10 minutes longer until firm and golden, cool completely before filling it. (Note: Don't be tempted to skip the blind baking step, otherwise your tart shell will shrink. I've learned the hard way.)

Before serving whip the double cream and creme fraiche with icing sugar, vanilla bean seeds and rum until soft peaks form. Mix it with broken meringue nests and fill your tart with the mixture. Top with mixed berries.