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. Cut in using a pastry cutter or fingertips until mixture resembles coarse crumbs with some pea-sized pieces remaining.
04 - In a separate bowl, whisk together heavy cream, egg, lemon juice, and vanilla extract until combined.
05 - Pour wet ingredients into dry ingredients. Fold gently with a spatula just until dough comes together; do not overmix.
06 - Turn dough onto lightly floured surface. Pat into a 7-inch circle, about 1 inch thick.
07 - Cut circle into 8 equal wedges. Transfer wedges to prepared baking sheet, spacing 2 inches apart.
08 - Brush tops with additional heavy cream. Bake for 16-18 minutes until golden brown. Cool on wire rack for 10 minutes.
09 - Whisk powdered sugar with lemon juice until smooth and pourable. Add more juice if needed to reach drizzling consistency.
10 - Drizzle glaze generously over cooled scones. Allow glaze to set for 5 minutes before serving.