Preheat oven to 400°F. Prepare a baking pan by lining with parchment paper and set aside.
In the bowl of a food processor fitted with a dough blade, combine flour, sugar, baking powder and salt. Add diced butter. Cover and pulse until the butter is chopped into the flour {the flour will resemble wet sand.}
In a measuring cup with a pouring spout, whisk together buttermilk, lemon zest, lemon juice, egg yolk, vanilla and poppy seeds.
Open the feed spout on the lid of the food processor and slowly pour in the liquids while pulsing. The dough will begin to come together and eventually form a ball. Do not over pulse.
Turn the dough out onto a lightly floured work surface, pressing it together to form a uniform piece of dough. Quickly flatten with your hands and pat the dough into approximately an 8 inch square.
Using a sharp knife, cut the dough into nine equal pieces, then cut each square in half diagonally. (You can also punch these out using a biscuit cutter.) Place each scone on the prepared baking sheet and brush with a little buttermilk, then sprinkle with sanding sugar if desired.
Bake for about 15 minutes or until the scones are baked through and slightly golden.Remove from oven and let cool on a baking rack.