Preheat oven to 350 degrees F. Coat a 9-inch x 13-inch baking dish with non-stick cooking spray.
In a large bowl, beat together ½ cup melted butter, sugar, and 1/2 cup brown sugar. Add eggs, vanilla, and milk. Mix to combine. Scrape sides of bowl as needed.
In a medium bowl, mix together the flour, baking powder, salt, and 1/2 teaspoon cinnamon. Add dry ingredients to wet ingredients and mix until just combined. Pour batter into prepared baking dish.
In a small bowl, mix remaining 1/2 cup brown sugar and 1 teaspoon cinnamon. Sprinkle over the top of the coffee cake. Cut remaining ½ cup butter into small pieces. Place the butter pieces all over the top of the cake.
Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean. The cake should be set and a crust will have formed on the top. Carefully remove coffee cake from the oven and allow to cool for 10 minutes. Serve warm or at room temperature.
If you make this coffee cake in an 11-inch x 15-inch baking dish, cook for 30-35 minutes, or until a toothpick inserted in the center comes out clean.