Place the flour, cornstarch, sugar, baking powder, and salt in the bowl of a food processor. Pulse 5-10 times, or until well combined.
1¾ cups all-purpose flour, ¼ cup cornstarch, ⅓ cup granulated sugar, 1 tablespoon baking powder, ½ teaspoon kosher salt
Add the cubed butter and pulse until the butter is the size of peas, about 10 pulses.
½ cup unsalted butter
In a large bowl, whisk together the heavy cream, egg, and vanilla. Add the butter-flour mixture to the wet ingredients and mix until the dough begins to come together into large clumps. If using any add-ins, add them to the dough now.
¾ cup heavy cream, 1 large egg, 1 teaspoon pure vanilla extract, 1 cup raspberries, 1 cup blueberries, 1 cup chocolate chips, 1 cup shredded cheddar cheese
Turn the dough out onto a lightly floured surface and gently shape it into an 8-inch round circle. Cut into 8 equal-sized triangles.
Place the scones on a parchment-lined baking sheet at least 2 inches apart. They will spread a little while baking.
Place the scones in the freezer to chill for 30 minutes. While the scones chill, preheat your oven to 350°F.
When the oven is fully preheated, brush the scones with egg wash and sprinkle them with coarse sugar, if using. Bake for 22-25 minutes, or until the scones are puffed and golden brown around the edges.
1 large egg, 1 teaspoon water
Allow the scones to cool completely before enjoying.