Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In the bowl of a stand mixer, whisk the flour, sugar, baking powder, and salt. This will not work with hand beaters, but can be done in a food processor instead of a stand mixer.
Add very cold butter to the flour mixture and mix on medium-low (or pulse if using food processor) until the mixture becomes fine crumbs.
In a separate small bowl combine the egg, cream, vanilla, and lemon zest. Add to the flour mixture and mix on low (or pulse) until the dough comes together, adding more cream 1 tbsp at a time if needed, until it is moist and mostly holds together (see picture).
Turn the dough onto a lightly floured surface and knead 4-5 times. Then use your hands to pat into a rectangle ½ inch thick. It should be roughly 11 inches long and 8 inches wide.
Using a sharp knife or bench scraper, cut the scones into 16 triangles. First cut the dough in half lengthwise, then in half widthwise. Cut each of the 4 sections into 4 triangles (see picture).
Place on prepared sheet pans and bake for 15-18 minutes, or until lightly golden on the bottom and the tops look dry.
While the scones are baking, combine the glaze ingredients in a bowl.
Once finished baking, allow scones to cool for 5-10 minutes on the sheet pan. Then place on a cooling rack. Once cool, drizzle with glaze and serve.