Preheat the oven to 350°F. In a large bowl, beat the melted butter and granulated sugar until they're smooth, two to three minutes. Beat in the eggs, one at a time, until the mixture is fluffy and pale in color. Beat in the vanilla extract and milk until incorporated. Editor's Tip: A handheld mixer or stand mixer with a whisk attachment makes quick work of beating the ingredients, but a regular whisk and some elbow grease will also do the job!

Mix the flour, baking powder and salt in a separate bowl. Add the dry ingredients to the wet ingredients and whisk just until they're incorporated. Pour the batter into a greased 13x9-inch baking dish. Editor's Tip: Scrape the sides of your bowl with a rubber spatula to help incorporate the dry and wet ingredients. Avoid overmixing; it'll make your cake tough.

In another large bowl, cream the softened butter, brown sugar, flour and cinnamon until they're well combined and creamy. Spoon dollops of the cinnamon mixture evenly over the cake batter. Use a knife to swirl the cinnamon filling through the cake batter.

Bake the cake until a toothpick inserted into the center comes out clean, 35 to 40 minutes.

Place the softened butter and cream cheese in a large bowl. Use a handheld or stand mixer to beat them together at medium speed until they're smooth, three to four minutes. Reduce the mixer speed to medium-low, add the confectioners' sugar and mix until it's incorporated, one to two minutes longer. Increase the mixer speed to medium and add the milk, vanilla extract and salt. Beat until the icing is fluffy, three to four minutes longer. Spread the icing over the warm cake. Serve it immediately or at room temperature. Editor's Tip: Increase the amount of milk you add to the icing to make it thinner and more of a drippy glaze.