Preheat oven to 220 °C (425°F) conventional (see note 2 if using a convection oven with a fan) and line a sheet tray with parchment paper. Set aside.
In a large bowl combine flour, cornstarch, baking powder, baking soda, salt and sugar. Using a whisk, mix until well combined.
Add in the cold, cubed butter, and using your fingers press the butter into the dry ingredients until the mixture resembles coarse crumbs with small bits of butter scattered throughout. The dry ingredients should become a pale yellow color.
Add in the buttermilk and using a fork, fluff the mixture together so that you end up with a shaggy dough. It's important not to overmix, so as soon as your mixture resembles a shaggy dough stop there.
Turn the dough out onto a floured surface, and using lightly floured hands, begin to bring the dough together until you form a rectangle that is ¾ of an inch high. Do not overwork or overmix the dough.
Coat the ends of a 2 ½ inch cookie cutter or glass (that has a thin lip - a thick lip won't work well) in flour, and use it to cut out the biscuits. Do not twist when cutting out the biscuits, just push straight down. Bring together any excess dough and cut more biscuits out. You should end up with about 7 biscuits (see note 4 if you want to do this step by hand).
Place the biscuits onto the prepared tray closely together - about ½ an inch apart and sprinkle the tops with white granulated sugar. Bake for 15 minutes or until the tops are golden brown. For best results, the dough needs to be cold before baking, so if you feel the butter has softened up a bit while preparing the dough, then freeze for 10 minutes before baking.
Allow to cool on the tray for 20 minutes before assembling. While the bisuits are cooling prepare the whipped cream.