In a bowl, whisk the flour, cornstarch, baking power, kosher salt, poppy seeds and sugar. Place the bowl in the freezer.
Mix together the heavy cream, large egg, vanilla, lemon zest (from one large lemon, but save aside 1 teaspoon for the glaze) and lemon juice in a spouted measuring cup. Place the measuring cup in the refrigerator.
Prepare a baking pan with parchment paper and set aside.
Remove the bowl of dry ingredients from the freezer and remove the heavy cream mixture from the refrigerator and give it another stir to ensure everything is evenly combined. Slowly drizzle the cream mixture over the dry ingredients while gently mixing with a fork. You want a wet dough with no dry flour at the bottom of the bowl, but you don’t want something as wet as muffin batter. Avoid overworking the dough, which will result in tough scones. Once all the cream is mixed in, use your hands to gently gather the dough into a ball; if the shape holds you are good. If the mixture is too dry to hold together, add a little heavy cream, a teaspoon at a time until the dough comes together.
Divide the dough into two equal portions. On a lightly floured surface, gently shape each piece of dough into a rough 5x7 inch rectangle.
Transfer both rectangles to the prepared baking sheet and freeze for 30 minutes or refrigerate for one hour. (You can also cover and refrigerate the dough overnight and bake the next morning).
20 minutes prior to baking the scones, preheat the oven to 400°F.
Remove the dough from the freezer (or refrigerator) and with a sharp knife cut each rectangle into 6 equal squares.
Place the squares back on the baking pan about 1-2 inches apart. Brush the tops of each scone with heavy cream (1 - 2 tablespoons), then sprinkle with coarse sugar.
Bake for 15-20 minutes or until just browning at the edges and no wet dough remains in the center.
Allow the scones to cool about 15 minutes before glazing.