Bake cake. Preheat oven to 350°F. Spray a 9x13-inch baking dish with nonstick spray. Prepare cake according to package directions. Pour into pan and bake according to package directions, or until a toothpick comes out clean. Let cool about 15–20 minutes.
Make filling. Whisk together the sweetened condensed milk, 4 Tablespoons melted butter, 1/4 cup brown sugar, 1 1/2 teaspoons cinnamon, 1 teaspoon vanilla and pinch of salt until smooth.
Poke and fill. Using the handle of a wooden spoon (about 1/4 inch), poke holes about 1 inch apart over the cake, going most of the way through. Slowly pour the cinnamon mixture over the cake. Use a spatula to spread it evenly and help the filling settle into the holes.
Make frosting. Beat the 4 ounces cream cheese and 4 Tablespoons butter until smooth. Add 1 3/4 cups powdered sugar and 1 teaspoon vanilla. Beat until creamy, adding milk 1 Tablespoon, if it's too thick.
Frost and chill. Spread frosting over the cooled cake. Refrigerate for at least 1 hour before serving.
I used a 15.25-ounce Pillsbury cake mix. Duncan Hines and Aldi mixes are also 15.25 ounces, while Betty Crocker has reduced its box size. Be sure to check the package before getting started.
Keep left over cake covered and stored in the refrigerator up to 2 days.Do not freeze.