Preheat Oven: Preheat your oven to 325°F (163°C).

Prepare the Pancake Batter: In a large mixing bowl, combine the flour, baking powder, granulated sugar, eggs, salt, milk, and half of the melted butter. Stir until the batter is smooth and well mixed.

Grease the Sheet Pan: Pour the remaining half of the melted butter into a half sheet pan (approximately 18×13 inches). Use a pastry brush to evenly coat the bottom of the pan.

Pour and Spread Batter: Pour the pancake batter into the prepared sheet pan. Use a spatula to spread it evenly across the pan.

Make the Cinnamon Swirl Mixture: In a small bowl, combine the melted butter, granulated sugar, brown sugar, and cinnamon. Mix well until smooth.

Add the Cinnamon Swirl: Spoon dollops of the cinnamon mixture across the pancake batter. Use a butter knife to gently swirl the mixture throughout the batter, creating a marbled effect.

Bake: Place the sheet pan in the preheated oven and bake for 15 minutes, or until the pancake is set and lightly golden on top.

Prepare the Icing: While the pancake bakes, whisk together the melted butter, powdered sugar, and milk in a small bowl to make the icing.

Finish and Serve:

Once the pancake is done baking, remove from oven and immediately drizzle with the icing. Slice into squares and serve warm.