Preheat the oven to 350 degrees. Mix the flour, sugar, baking powder, and salt. Add the eggs, vanilla, milk, and 1/2 c. melted butter, and mix well. Pour into a greased loaf pan. Melt the second 1/2 c. butter, and stir in the cinnamon, flour, and brown sugar. Pour on top of the batter and swirl through with a butter knife, making sure to get all the way to the bottom. Bake for 50-60 minutes, or until done through.