Pulse together flour, sugar, salt, and lemon zest in a food processor.
Cut butter into 1/2" cubes and pulse it into the flour mixture until the mixture reaches a coarse sandy texture.
Add the egg, vanilla, and poppy seeds and pulse just until the dough comes together.
Turn dough out onto a lightly floured surface. Cut the dough into three sections and smoosh each section down and away from you with the palm of your hand.
Gather the dough back together with a bench scraper and form it into a disc. At this point, the dough can be wrapped and chilled, or frozen to use later.
Grease a 9" fluted tart pan with butter then press the dough into the pan and up the sides into one even layer.
Chill for 45 minutes or freeze for about 15 minutes until firm. Meanwhile preheat the oven to 375˚F (190˚C).
Prick the dough all over with a fork then place a piece of parchment over the dough and fill it up fully with pie weights or beans.
Blind bake the crust for 20 minutes with the pie weights. Then remove the pie weights and bake for another 15-20 minutes until the edges start to brown and the bottom center is cooked through.
Remove from the oven and continue on with filling it as instructed below or let it cool then wrap it to use later on.