Preheat oven to 400 F. (200 C.), line a baking sheet with parchment paper or Silpat, and chop 1 cold stick (4 oz.) of unsalted butter into smaller pieces so it can warm slightly while you prepare your other ingredients.
In a large mixing bowl, combine 2½ cups flour, 1 tsp. baking powder, 1 tsp. baking soda, ½ tsp. kosher (or flake) salt, ⅓ cup sugar, 1 tsp. poppy seeds and ½ tsp. lemon zest. Mix gently. Set aside.
In a separate small mixing bowl, combine 1 beaten egg, ⅓ cup lemon juice, ¼ cup heavy cream and 1 tsp. real vanilla extract. Set aside. (You’ll add these ingredients to your dough after you’ve mixed in the butter.)
Add your semi-softened butter to the large mixing bowl. Use your hands to mix it into the flour until you have a crumbly mixture, allowing a few bits of butter to stay unmixed. This allows nice pockets of butter to form in your scones.
Add the contents of your small mixing bowl to the large one, and mix with a spoon until thoroughly combined into a sticky dough.
Get your hands nice and floury to make it easier to pick up and roll dough with your hands. You can either loosely roll the scones into a gently flattened ball with 2 heaping tablespoons of dough, or make a larger ball and use a round cookie cutter (or wine glass like I’ve done) to cut them perfectly into thick disks.
Place on the baking sheet about an inch apart, and bake scones for 15–20 minutes on the bottom rack. When done, cool for 10 minutes before serving.