This is one of the rare recipes where I use Earth Balance Buttery Sticks instead of the spread, and you want to use it cold right out of the fridge. Cut the stick into eight pieces and add them to your mixing bowl.
Mix on medium speed until the Earth Balance is smooth.
Add the sugar and vanilla. Mix on medium-low speed for several minutes, scraping down the bowl as needed. Add the plant milk and beat again.
Add one-third of the flour mixture, and beat it in. Scrape off the beaters into the bowl.
Add half of the remaining flour mixture, then stir it in with a spatula. Add the rest of the flour mixture, and stir it in as well.
Spray a nine-inch tart pan with a non-stick spray. (Be sure to use a pan that has a removable bottom.) Use a piece of paper towel to spread the spray into every bit of the pan. Failure to do so may lead to your crust sticking to the pan and making it impossible to remove the tart shell intact.
Add the shortbread dough, and spread it out evenly on the bottom and sides of your tart pan. Be sure that your dough does not get pressed too thin on the fluted sides.
Bake for 15 to 17 minutes, until your crust is lightly golden brown. If your crust has any puffy spots, you have a minute or two when you first take the crust out of the oven when it is still soft enough to push it into the shape you want. Be careful not to burn your fingers!
Let the crust cool completely before removing it from the sides of the pan. Gently push up on the removable bottom and remove the ring. Leave the crust on the bottom of the pan for now. The edges should come away from the sides without breaking.
Fill your baked tart shell according to your recipe. Chill if your recipe requires it, then slice your tart into quarters. Carefully remove the quarters from the bottom of the tart pan before cutting into smaller pieces. Serve your tart and enjoy!