Using a hand mixer or stand mixer with the paddle attachment, combine the flours and cut up butter, mixing on low speed. Add the sugar and increase speed to medium and continue mixing.
When the butter is in very small crumb pieces, add the egg yolk and the vanilla. Then add the cream while mixing. Add ½ tablespoon of cold water and mix for 30 seconds until the dough begins to pull together.
Turn off the mixture and press together with your hands. If the dough is still crumbly, add up to ½ tablespoon more of water and mix for 10 seconds.
Remove the dough from the mixer and divide into two equal balls of dough. Press each dough ball into a disc about 1/2" thick. Wrap each disc in cellophane or a ziploc bag. Chill for for 1-2 hours. Makes 2 tart crusts.**At this point you can also freeze one disc of dough for another day. Preheat the oven to 375 degrees, with the rack in the center of the oven.
Prepare the tart pan with a piece of parchment to line the bottom of the pan only.
Roll the dough out to ⅛” thick between two lightly floured parchment sheets. When the dough is the approximate size, pull off the top parchment and set the dough side down into the tart pan. Remove the excess dough by using your fingers to press against the tart edges and pull the excess dough off.
Poke holes in the crust using a fork. Brush 1 tablespoon egg white on each crust.
Bake for 22-25 minutes, until golden brown. Remove from the oven.
Pour the hot filling into the baked crust.