In a large bowl, mix together flour, sugar, salt, and baking powder. Whisk to combine.
Next, use a box grater to grate the frozen butter into the dry ingredients. Combine the two mixtures using a pastry cutter or your fingers until the flour resembles a pea-sized crumb and is very dry. Place in the refrigerator to keep cool.
In a medium-sized bowl, mix together 1/2 heavy cream, the egg, and vanilla. Remove the flour and butter mixture from the refrigerator and mix the wet ingredients into the dry. Add in any mix-ins (such as blueberries, lemon zest, chocolate chips, etc.) during this stage. Mix until combined. The mixture should be firm, but still moist and sticky.
Pour the dough onto a floured surface and form into a loose ball. Don't worry if the dough is still sticky. If it looks a bit too sticky, add in a little more flour. However, if it seems too dry, add in 1-2 Tablespoons of heavy cream.
Shape the ball into a 8-inch disc. Using a knife or a bench scraper cut the dough. For large scones, aim for 8 wedges. For smaller scones, split the dough into two 5-inch discs and cut into 8 wedges.
Place scones on a prepared baking sheet with parchment about 2-3 inches apart. Brush scones with the remaining heavy cream or buttermilk. This will give the scones an extra crunch when baking. Place the prepared scones in the refrigerator or freeze for 15 minutes.
While the scones are sitting in the fridge, preheat the oven to 400°.
After 15 minutes, place the scones in the oven for about 18-24 minutes. Keep an eye on the scones are remove them from the oven when they form a golden brown ring around the edge. The larger scones may take 25-30 minutes to bake.