In a medium bowl, melt butter, then stir in brown sugar, cinnamon, salt, and vanilla until smooth.
Pour ⅔ of the goo into an 8×8 2 quart pan and heat in a 450°F oven until it's starting to bubble (4–5 minutes). Reserve remaining goo.
Whisk flour, salt, baking powder, and sugar in a large bowl.
Add buttermilk and vanilla; stir just until combined (thick and lumpy is good).
Pour batter over hot goo (do not stir).
Drizzle remaining goo on top and gently swirl.
Sprinkle cinnamon sugar evenly and score into 9 squares.
Bake 20–26 minutes, rotating once, until golden and bubbling. Tent if needed. Note: if pan is full, place sheet with aluminum foil underneath to catch drippings.
Whisk powdered sugar, milk, vanilla, and salt until smooth.
Cool biscuits 5–10 minutes, drizzle with glaze, and serve warm.
