Preheat oven to 450°F. Pour melted butter into an 8x8-inch baking dish.
In a large bowl, whisk together flour, baking powder, sugar, and salt.
Pour in the buttermilk and stir just until combined. Do not overmix.
Transfer the dough to the buttered dish and press evenly to the edges.
Use a knife to cut the dough into 9 squares before baking.
In a small bowl, mix brown sugar and cinnamon, and sprinkle it over the dough.
Bake for 25-28 minutes or until golden and baked through.
Let cool slightly before serving. Serve warm for the best experience.