01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together flour, sugar, poppy seeds, baking powder, and salt until well combined.
03 - Add cold, cubed butter to the dry ingredients. Using a pastry cutter or your fingertips, cut in the butter until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - In a separate bowl, whisk together cream, egg, lemon zest, lemon juice, and vanilla extract until blended.
05 - Pour wet ingredients into dry ingredients. Stir gently with a fork until just combined. The dough should be slightly sticky but manageable. Do not overmix.
06 - Turn dough out onto a lightly floured surface. Pat into a 1-inch thick circle about 8 inches in diameter. Cut into 8 wedges and transfer to the prepared baking sheet, spacing them 2 inches apart.
07 - Brush the tops of scones with a thin layer of heavy cream for a golden finish.
08 - Bake for 16-18 minutes, or until scones are golden brown and a toothpick inserted into the center comes out clean. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack.
09 - While scones cool, whisk powdered sugar with 2 tablespoons lemon juice in a small bowl. Add more juice, 1 teaspoon at a time, until you reach a thick but pourable consistency. Stir in lemon zest if using.
10 - Drizzle glaze over cooled scones in a back-and-forth motion. Allow glaze to set for 10-15 minutes before serving.