Preheat the oven to 450 degrees.
Cut the butter into small cubes and place in the freezer.
In a medium-sized bowl, whisk the flour, sugar, baking powder, and salt together.
Add the butter to the dry ingredients and working quickly, and using your fingers, rub the butter into the flour until the flour resembles coarse meal.
Add the cream and the mascarpone and using a rubber spatula or wooden spoon, gently incorporate the wet ingredients into the dry. The mixture will seem too dry and you'll be dying to add more cream. Resist and keep stirring, or, do as I do, and knead the dough in the bowl with your hands until it comes together.
Dump the dough out on the counter (you shouldn't need to flour your work surface, as the dough will still be dry-ish) and knead the dough a few times until it is no longer crumbly and the dry and moist bits are fully integrated.
Lightly flour your work surface, if the dough is at all sticky, and pat the dough into a rectangle a little less than 1/2 of an inch thick. Be super gentle as you pat. Using a 2 1/2" biscuit cutter, dipped in flour, begin cutting out shortcakes from the rectangle. Do not twist the cutter as you push through the dough (if you twist as you push down or pull up, the biscuits will not rise as well).
Place the shortcakes on a parchment lined baking sheet, about 2 inches apart. Continue cutting out cakes until you have used up all of your dough, re-forming your rectangle with scraps, as you go.
at this point, you can freeze the shortcakes, still on the tray and tightly wrapped in plastic wrap, for up to two weeks.
Brush the tops of the shortcakes with melted butter. Place one cake on top of another and sprinkle the top with sanding sugar. Place the tray in the pre-heated oven and bake for 10 to 12 minutes, rotating after 5, and checking after 10. The cakes are ready when they are golden brown on top and lightly browned on the bottom. Make the whipped cream while the shortcakes bake.