Preheat oven to 375 degrees. Coat a 24 mini-muffin pan with cooking spray. Mix 1/2 cup sugar and 1/4 cup butter in a large bowl. Stir in the milk and vanilla, then mix in the flour, baking powder, and salt until just combined. Fill the prepared mini muffin cups about half full. Bake in the preheated oven until the top of the muffins are lightly golden, 15 to 20 minutes. While muffins are baking, place 1/4 cup of melted butter in a small bowl. In a separate bowl, combine 1/2 cup of sugar and cinnamon. Turn the muffin tin pan over to release the muffins on a plate or cooling rack. Dip each muffin in the melted butter, and roll in the sugar-cinnamon mixture. Let cool and serve.