Preheat the Oven:
Preheat your oven to 400°F (200°C). Spray a 9 x 13-inch casserole dish with cooking spray and set aside.
Prepare the Pancake Base:
In a small saucepan, combine the water and butter. Heat over medium until the mixture comes to a boil. Reduce the heat to low and add the baking mix, stirring quickly for about a minute until the mixture forms a ball.
Remove the pan from the heat. Stir in the eggs one at a time, ensuring each egg is fully incorporated before adding the next. Continue stirring until the batter is smooth and well combined.
Spread the batter evenly into the bottom of the greased casserole dish, ensuring not to spread it up the sides.
Make the Cinnamon Swirl:
In a small bowl, mix together the melted butter, light brown sugar, granulated sugar, and cinnamon until well combined. Dollop this mixture over the batter in the casserole dish. Use a knife to swirl the cinnamon mixture through the batter, creating a marbled effect.
Bake:
Bake in the preheated oven for 25 to 30 minutes, or until the sides are golden brown and the pancake is puffed up.
Prepare the Icing:
While the pancake bakes, whisk together the melted butter, powdered sugar, milk, and vanilla extract until smooth and well combined.
Finish and Serve:
Once the pancake is done baking, remove it from the oven and immediately drizzle the prepared icing over the top. Slice the pancake into squares and serve warm.