01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish with nonstick spray or butter.
02 - In a large mixing bowl, combine cake mix, eggs, milk, vegetable oil, and vanilla extract. Beat with a hand mixer or stand mixer on medium speed for about 2 minutes until smooth. Pour the batter into the prepared baking dish and smooth the surface.
03 - Bake the cake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
04 - While the cake bakes, mix the brown sugar and ground cinnamon together in a small bowl. Stir in melted butter to form a thick paste.
05 - Remove the baked cake from the oven. Immediately use the handle of a wooden spoon to poke holes all over the surface, spaced about 1 inch apart.
06 - Spoon or drizzle the cinnamon filling evenly over the warm cake, allowing it to seep into the holes. Let the cake cool completely in the pan.
07 - In a medium bowl, beat softened cream cheese and butter together until smooth. Add powdered sugar, milk, and vanilla extract, mixing until the frosting is creamy and smooth.
08 - Once the cake is completely cool, spread cream cheese frosting evenly over the top. Cut into squares and serve.