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 smooth.
05 - Pour wet ingredients into dry ingredients. Stir gently with a fork until just combined. Do not overmix—the dough should be slightly shaggy.
06 - Turn dough out onto a lightly floured surface. Gently pat into a 1-inch thick circle, approximately 8 inches in diameter. Cut into 8 equal wedges and transfer to the prepared baking sheet, spacing 2 inches apart.
07 - Brush scones lightly with extra cream. Bake for 16-18 minutes, or until golden brown and a toothpick inserted in the center comes out clean. Transfer to a wire rack to cool completely.
08 - Whisk powdered sugar with 2 tablespoons lemon juice until smooth. Add additional juice, 1 teaspoon at a time, until desired pouring consistency is reached. Stir in lemon zest if using.
09 - Drizzle glaze generously over completely cooled scones. Allow glaze to set for 10-15 minutes before serving.