Combine sugar and lemon zest. In a large bowl rub together the sugar and lemon zest with your fingertips. This will help release the oils of the lemons.
Add dry ingredients. Whisk in the flour, baking powder and salt. Stir in the poppyseeds.
Add butter. Cut in butter with a pastry blender or rub in with your fingers tips, until the mixture has the texture of coarse meal
Add wet ingredients. In a small bowl or measuring cup, Whisk together heavy cream, egg, lemon juice and vanilla. Make a well in the dry ingredients, and pour in the heavy cream mixture. Stir lightly with a wooden spoon or spatula until dough comes together. Dough will be sticky and shaggy looking.
Knead dough. Turn out dough onto a lightly floured surface, and gently knead dough together a few times. I press down on the dough into about 1/2" thickness, then fold in half. I repeat this about 3-4 times. Do not overwork the dough, you just want to incorporate the flour and bring it together and knead a few times to help create lofty layer.
Shape scones. For best shape, I like to line an 8 inch cake pan with plastic wrap and then press dough into the pan. Cover with plastic wrap. Refrigerate for 20 minutes.
Prep oven and pan. Preheat the oven to 425F. Line a baking sheet with parchment.
Shape scones. Once the oven is preheated, remove the pan from the fridge, and remove the scone dough from the pan. Turn the dough upside down onto a lightly flurry surface and peel away the plastic wrap and Cut into 8 equal wedges using a sharp knife. Transfer to a baking sheet spaced a few inches apart in two equal rows. Brush tops with a little extra heavy cream.
Bake the scones. Bake until golden brown and cooked through, about 18-20 minutes. Remove the pan from the oven and cool on the pan for 5 minutes then Transfer scones to a wire rack to cool completely.
Make lemon icing. Combine icing ingredients in a bowl and whisk to combine. Adjust consistency if needed. Drizzle cooled scones with icing. Decorate tops with a sprinkle of poppyseeds.