Heat oven to 400°F. Line a large baking sheet with parchment paper.
Whisk flour, sugar, baking powder, salt, poppy seeds, and lemon zest in a large bowl.
Cut in the cold butter with a pastry cutter or fingers until pea-sized bits remain.
Stir heavy cream, lemon juice, and vanilla in a small bowl.
Pour wet ingredients into dry. Mix gently until a shaggy dough forms; do not overwork.
Turn dough onto a lightly floured surface. Pat into a 7-inch round about 1 inch thick.
Slice into 8 wedges and arrange on the prepared sheet, spacing slightly.
Brush tops with cream and sprinkle with coarse sugar.
Bake 16–18 minutes, until golden at the edges and set in the centers. Cool 10 minutes.
Whisk powdered sugar, lemon juice, and cream until smooth. Drizzle over warm scones and serve.