Preheat the oven to 425ºF. Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, cornstarch, baking powder, baking soda, sugar, and salt. Add the butter and, using a pastry cutter, cut the butter into the dry ingredients until the mixture resembles coarse sand with a few pea-sized clumps of butter intact.
Add sage, parmesan cheese, and ¾ cup of the buttermilk and stir with a rubber spatula until the dough comes together into a shaggy mixture. Add the remaining ¼ cup buttermilk, a little at a time, until the dough is moistened.
Using a cookie scooper, scoop 8 biscuits and place onto the prepared baking sheet, leaving 2-inches between each or shape with a biscuit cutter. Brush the tops of the biscuits with melted butter, and sprinkle with flaky salt. Place biscuits in freezer for 10 minutes.
Remove from freezer and bake for 17 to 20 minutes, until the biscuits are lightly golden.
Serve warm with guava butter. Enjoy!
