Preheat oven to 450 degrees.
Add flour, baking powder, sugar and kosher salt to a bowl. Whisk until fully combined.
Pour in the buttermilk and bring the batter together with a spatula. Once there are no dry spots in the dough and everything has been incorporated, pour in the blueberries and mix again.
Line a 9x9 baking dish with parchment paper and pour in a stick of melted salted butter.
Add the batter to the buttery pan and spread out evenly with a spatula. Youll have butter seep over the dough in areas, that's okay.
Cut the dough into 9 equal squares.
Bake for 40 minutes uncovered in the middle rack.
Toward the end of baking, make the glaze on the stove over medium heat. Add the milk and vanilla to the pan and once warm add the powdered sugar. Whisk or use a spatula to mix the glaze until there are no lumps.
The biscuits should be golden brown on top and a toothpick will come out clean when done. Pour the hot glaze over the hot biscuits. Let sit for 20 minutes then enjoy while warm!