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 well combined.
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 some pea-sized pieces remaining.
04 - In a separate bowl, whisk together the heavy cream, egg, lemon juice, and vanilla extract until fully blended.
05 - Pour the wet ingredients into the dry mixture. Stir gently with a spatula or wooden spoon until just combined. The dough should be slightly shaggy but hold together. Do not overmix.
06 - Turn the dough out onto a lightly floured surface. Gently knead 3-4 times to bring the dough together. Pat into a 1-inch thick circle, about 7 inches in diameter.
07 - Cut the circle into 8 equal wedges using a sharp knife or bench scraper. Transfer wedges to the prepared baking sheet, leaving 2 inches of space between each scone.
08 - Brush the tops of each scone lightly with additional heavy cream for a golden finish.
09 - Bake for 16-18 minutes until golden brown on top and a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool completely.
10 - While scones cool, whisk together powdered sugar and lemon juice in a small bowl until smooth. The glaze should be thick but pourable; add more lemon juice if needed.
11 - Drizzle the glaze generously over the completely cooled scones. Allow glaze to set for 10-15 minutes before serving.