Preheat Oven: Preheat the oven to 350F.
Mix Ingredients: Add the butter, granulated sugar, and brown sugar to a large mixing bowl. Beat until light and fluffy.
Combine Dry and Wet Ingredients: Add the eggs, vanilla, and milk. Continue to beat until everything is well combined. In a separate bowl, whisk the flour, baking powder, and salt together. Add the flour mixture to the wet ingredients and mix just until combined.
Make Streusel Topping: Mix the flour, brown sugar, and cinnamon for the streusel topping together in a small bowl.
Assemble: Pour half the batter into a greased 9x13 baking pan. Sprinkle 1/3 of the streusel mixture evenly over the batter, then pour the remaining batter over top. Sprinkle the remaining streusel mixture evenly over the top. Drizzle the melted butter over the topping.
Bake: Bake for 35-40 minutes, or until an inserted toothpick comes out clean with no white batter (It may come out with some cinnamon and brown sugar.)