Preheat oven to 425F. Lightly grease a 12" cast iron skillet or large round baking pan w/ butter.
Biscuits: 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 w/ a wooden spoon until barely combined. Then, use your hands to knead & fold the dough over itself a couple of times until combined.
Add the blueberries; gently fold until the blueberries are evenly dispersed. The dough will be a little sticky, but should be soft enough to scoop!
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 w/ buttermilk or heavy cream. Then, sprinkle the tops w/ the cinnamon sugar.
Bake for 20-25 mins, or until fluffy & golden.
Vanilla Bean Butter: In a bowl, whip the butter & vanilla bean paste together with a whisk or handheld electric mixer until combined & fluffy.
Serve the biscuits warm w/ the whipped salted vanilla bean butter, honey & flaky sea salt if desired.
