01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together the flour, sugar, poppy seeds, baking powder, baking soda, salt, and lemon zest until evenly distributed.
03 - Add the cold, cubed butter to the dry mixture. Cut in the butter using a pastry cutter or your fingertips until the mixture resembles coarse crumbs with pea-sized pieces of butter remaining.
04 - In a separate bowl, whisk together the heavy cream, egg, lemon juice, and vanilla extract until well combined.
05 - Pour the wet ingredients into the dry mixture and stir with a spatula until just combined. Do not overmix—some dry spots are acceptable.
06 - Turn the dough out onto a lightly floured surface and gently knead 3-4 times to bring it together. Pat into a 1-inch thick circle.
07 - Cut the circle into 8 equal wedges and place them on the prepared baking sheet, spacing about 2 inches apart.
08 - Brush the tops of each scone with a thin layer of additional heavy cream.
09 - Bake for 16-18 minutes, or until golden brown on top and a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool completely.
10 - Whisk together powdered sugar and 2 tablespoons lemon juice until smooth. Add additional lemon juice 1 teaspoon at a time until desired consistency is reached.
11 - Drizzle the glaze over the completely cooled scones. Allow glaze to set for 10 minutes before serving.