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 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 butter pieces remaining.
04 - In a separate bowl, whisk together the heavy cream, egg, lemon juice, and vanilla extract until smooth.
05 - Pour the wet ingredients into the dry mixture. Stir with a spatula or wooden spoon until just combined. The dough should be slightly sticky but hold together. Do not overmix.
06 - Turn the dough out onto a lightly floured surface. Gently knead 3-4 times to bring it together, then 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. Place wedges on the prepared baking sheet, spacing about 2 inches apart.
08 - Brush the tops of each scone lightly with additional heavy cream to promote golden browning.
09 - Bake for 16-18 minutes until the tops are golden brown and a toothpick inserted into the center comes out clean. Transfer to a wire rack to cool completely.
10 - Whisk together powdered sugar and lemon juice, starting with 2 tablespoons and adding more as needed to reach a smooth, drizzling consistency.
11 - Drizzle the glaze over the completely cooled scones. Allow glaze to set for 10-15 minutes before serving.