Preheat oven to 425ºF (220ºC). Cut butter into a large dice. Keep in refrigerator until ready to use. Line a large, rimmed baking sheet with parchment paper and set aside. Melt 2 tablespoons of butter in a small saucepan or in the microwave. Set aside.
In a food processor, combine the flours, baking powder, sugar and kosher salt. Pulse briefly to combine. Add the butter and pulse until butter is the size of small peas. Pour flour mixture into a large bowl.
Add the buttermilk. Stir until a shaggy dough forms. (If the air is very humid, hold back about 1/4 cup buttermilk. Add more or less as needed.)
Flour a work surface and remove dough from bowl. Using a bench scraper or your hands, work and knead dough briefly to come together. Dust dough with flour if sticky. Once the dough is holding together well, pat down and fold in half. Pat down again and fold. Repeat one more time for a total of three folds.
Pat dough into a roughly 8 1/2" x 9" rectangle, aiming for square corners. Cut dough into nine squares with a bench scraper or knife. Place biscuits on the baking sheet, brush with melted butter and sprinkle with flaky sea salt. If time allows, place sheet pan in the refrigerator or freezer for 10 minutes to chill.
Bake for 15 minutes or until golden brown. Cool for 5 minutes and serve warm.