Preheat oven to 425F. Lightly grease a 12" cast iron skillet or large round baking pan with butter.
In a large bowl, whisk together the cake flour, all-purpose flour, sugar, baking powder, baking soda & salt.
Add in the cubed butter; use a pastry cutter or two knives to cut the butter into the flour mixture until crumbly & resembles pea-sized chunks.
Make a well in the center; pour in the buttermilk & sourdough discard. Mix with a wooden spoon until barely combined.
Use your hands to knead & fold the dough over itself a couple of times until combined.
Add the blueberries; gently fold until evenly dispersed.
Use a large cookie scoop or a ½ cup measuring cup to scoop each biscuit & place in the prepared skillet or pan.
Mix together the brown sugar & cinnamon in a bowl for topping.
Brush the biscuits with buttermilk or heavy cream. Sprinkle the tops with the cinnamon sugar.
Bake for 20-25 minutes, or until fluffy & golden.
In a bowl, whip the butter & vanilla bean paste together with a whisk or handheld electric mixer until combined & fluffy.
Serve the biscuits warm with the whipped salted vanilla bean butter, honey & flaky sea salt if desired.