01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together flour, sugar, poppy seeds, lemon zest, baking powder, baking soda, and salt until thoroughly blended.
03 - Add cold butter cubes to the dry mixture. Using a pastry cutter or your fingertips, work butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - In a separate bowl, whisk together heavy cream, egg, lemon juice, and vanilla extract until fully combined.
05 - Pour wet ingredients into the dry mixture. Stir gently with a fork just until the dough comes together. Do not overmix.
06 - Turn dough onto a lightly floured surface. Gently knead 2-3 times to bring together, then pat into a 1-inch thick circle. Cut into 8 equal wedges and transfer to the prepared baking sheet.
07 - Brush the tops of each scone with a thin layer of additional heavy cream.
08 - Bake for 16-18 minutes until scones are golden brown on top and a toothpick inserted into the center comes out clean.
09 - Transfer scones to a wire rack and let cool for at least 15 minutes before glazing.
10 - Whisk together powdered sugar and lemon juice until smooth and drizzle-consistency. Adjust with additional lemon juice if needed.
11 - Drizzle the glaze over the cooled scones. Allow glaze to set for 5 minutes before serving.