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 well blended.
03 - Add cold, cubed butter to the flour mixture. Use a pastry cutter or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
04 - In a separate bowl, whisk together heavy cream, egg, lemon juice, and vanilla extract until fully combined.
05 - Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. The dough should be slightly shaggy but hold together when pressed. Do not overmix.
06 - Turn the dough onto a lightly floured surface. Gently pat into a 7-inch circle, about 1 inch thick. Cut into 8 equal wedges using a sharp knife or bench scraper.
07 - Transfer wedges to the prepared baking sheet, spacing them 2 inches apart. Brush tops with a thin layer of heavy cream for golden browning.
08 - Bake for 16-18 minutes until golden brown and a toothpick inserted in the center comes out clean. Transfer to a wire rack and let cool completely.
09 - Whisk together powdered sugar and lemon juice until smooth and pourable. Add more lemon juice 1 teaspoon at a time if needed to reach desired consistency.
10 - Drizzle the glaze generously over the completely cooled scones. Allow glaze to set for 10-15 minutes before serving.