Cake: In a large bowl, whisk together flour, sugar, baking powder, and salt. In a separate bowl, whisk together milk, eggs, and vanilla extract. Pour wet ingredients into dry ingredients and whisk until mostly incorporated but a few dry streaks remain. Whisk in melted butter.
Cinnamon topping: Stir all ingredients together in a medium bowl.
Pour half of the cake batter into a greased 9x13-inch (22 x 33 cm) baking dish. Drop spoonfuls of cinnamon topping on the batter, using half of the topping. Use the tip of a butter knife to swirl the topping through the cake. Pour the rest of the batter on top. Spoon the rest of the topping on and swirl again.
Bake for 25–30 minutes or until a toothpick inserted into the center comes out clean (or with only a few crumbs attached). Remove from oven and let cool.
Frosting: Whisk together cream cheese, powdered sugar, and milk. If frosting is too thick, add more milk until it becomes a pourable texture. Drizzle cake with frosting. Slice and serve!