At least one hour before preparing dough (or the night before), place butter in freezer.
In a medium bowl, whisk together sour cream, egg, and vanilla. Set aside in fridge to keep cool.
In a separate large bowl, combine sugar and lemon zest, pressing zest into sugar to release oils.
Whisk in flour, baking powder, baking soda, and poppy seeds and mix well.
Grate in frozen butter and combine with dry ingredients. Using a pastry blender or a fork, blend butter into flour mixture.
Fold sour cream mixture into dry ingredients with a rubber spatula until just combined. Use floured hands to continue to lightly knead dough into a ball.
Place dough on a lightly floured surface and dust top of dough ball with a little more flour. Gently shape into a 8-inch round flattened disk and cut into 8 wedges. Place wedges at least 1-inch apart on a large baking sheet lined with parchment paper or a silicone baking liner.
Preheat oven to 400°F and place baking sheet with scones in the freezer to chill while it heats up.
Bake for 16-17 minutes or until scones are lightly golden on top and golden brown on bottom. Let cool before adding glaze.
Whisk glaze ingredients together until smooth.
Place scones on cooling rack over parchment paper or paper towels to catch drips. Drizzle glaze over cooled scones and allow to set.
Make Ahead: These lemon poppyseed scones can be frozen prior to baking -- just freeze them solid, then place in a freezer bag for up to 2 months. Then just bake straight from frozen and add a few more minutes to the bake time.