To make the scones: In a medium bowl, whisk together the sugar and lemon zest until the sugar turns slightly yellow and fragrant.
Sift in flour and baking powder, add kosher salt and poppy seeds, whisk to combine.
Add cold cubed butter to the flour mixture, work the butter into the flour mixture with your fingertips until all the butter pieces are flat.
Add egg, heavy cream and vanilla extract to a small bowl, whisk to combine. Set aside 2 tablespoons for egg wash. Make a well in the middle of the flour mixture and pour in the liquid mixture. Fold with a spatula or wooden spoon until a shaggy dough forms.
Turn the dough out onto a well-floured board or countertop, and pat the dough out into a rectangle about ½” thick. Fold the dough over itself and pat it out again, do this 2 - 3 times.
Shape the dough into a rectangle and roll it out to about 5” x 8” or 6” x 9”. Cut the rectangle in half lengthwise, then cut in three crosswise to make 6 squares. Cut each square diagonally to make 12 triangles.
Place the scones on a parchment-lined baking sheet and freeze for 15 minutes while you preheat the oven to 400°F.
When the oven is ready, brush the top and sides of the scones with reserved egg wash, and sprinkle generously with turbinado sugar. Bake for 15 minutes until golden brown.
Remove from the oven and allow to cool on the baking sheet 5 - 10 minutes before transferring the scones to the wire rack to cool completely.
To make the glaze: Whisk together softened cream cheese and lemon juice. Slowly add powdered sugar while whisking until a creamy glaze forms. Stir in some lemon zest at this point. Taste the glaze and add more sugar if you prefer this to be sweeter, this is a pretty tangy glaze. Spoon glaze over the cool scones, as much or as little to your taste.