Use food processor (seriously, do), to pulse the flour (all by itself) to get rid of any clumps. Then, add butter and pulse again until small crumbs form. Add the icing sugar and egg. Pulse until a ball is about to form, but not fully.
Take the dough out, form into a ball and place in the fridge for 30min-1hr.
Roll out the dough on a lightly floured surface, and line a tart shell with it. Poke with a fork to make holes at the bottom.
Place the pan in the freezer for 30min-1hr (you might want to wrap it in a freezer ziplock bag).
In the meantime, turn the oven on to 360F. You want it ready ahead of time, so that the heat will be stable.
Once frozen, line the pan with aluminum foil and pie weights. Bake for about 20min.
Lower heat to 350F.
Then, remove the aluminum foil and the weights, and bake for 10 more min, until dry and lightly golden.