How to Make It
Preheat oven to 200°F. Stir together brown sugar, cinnamon, and 1/4 cup room-temperature butter in a bowl until thoroughly blended. Spoon mixture into a ziplock plastic bag. Snip a 1/4-inch tip off 1 bag corner. Set aside.
Whisk together powdered sugar, vanilla, and 3 tablespoons of the milk in a bowl until smooth, adding up to 1 additional tablespoon of the milk as needed to reach desired glaze consistency. Set aside.
Stir together flour, baking powder, and salt in a large bowl. Whisk together egg and remaining 3/4 cup milk in a bowl; gradually stir into flour mixture. Gently fold in 1 tablespoon melted butter. (Batter will be lumpy.)
Heat a nonstick skillet over medium; coat with cooking spray. For each pancake, pour about 1/3 cup batter onto skillet. Pipe brown sugar-cinnamon mixture onto pancake in a spiral pattern, starting at center of pancake and ending at outer edge. Cook until top is covered with bubbles and edges appear slightly dry, 3 to 4 minutes. Flip; reduce heat to medium-low, and cook until pancake is fluffy and cooked through, about 2 minutes. Transfer pancake to a baking sheet; place in preheated oven to keep warm. Repeat process with remaining batter and brown sugar-cinnamon mixture, re-greasing skillet as needed. Drizzle cooked pancakes with glaze; serve.