Preheat oven to 375°. Line a baking sheet with a silicon mat and set aside.
In a large mixing bowl, whisk the flour, granulated sugar, baking powder, salt, lemon zest, and poppy seeds.
Cube the cold butter pieces and add to bowl. With a pastry cutter, cut the butter into flour mixture until the mixture resembles coarse sand. Put in fridge while mixing wet ingredients together.
In a small bowl, whisk the heavy cream, egg, and vanilla. Take dry ingredients out of the fridge and pour wet mixture over dry and mix until all the mixture is moistened.
Transfer the dough to a lightly floured surface and mix only until combined. If too wet add more flour, if too dry add more heavy cream. Shape dough into an 8 inch disk.
Cut dough into 8 even wedges. Place on baking sheet and put in the fridge for 15 more minutes. Bake for 20-22 minutes or until the edges are golden brown. Cool completely before glaze.