Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or non-stick spray.
In a large bowl, combine the cake mix, melted butter, eggs, vanilla extract, and cinnamon. Stir until a thick, sticky dough forms. Press the dough evenly into the bottom of the baking dish.
In a separate bowl, beat the softened cream cheese until smooth. Add melted butter, eggs, and vanilla extract, mixing well.
Gradually add powdered sugar and cinnamon, mixing until smooth. Pour the cream cheese mixture over the cake base and spread evenly.
Bake for 40-45 minutes, until edges are set and golden brown but center has a slight jiggle. Cool completely in the pan.
For the drizzle, beat softened cream cheese until smooth. Add powdered sugar and vanilla extract, then milk, 1 tbsp at a time, until smooth and drizzling consistency is reached.
Drizzle over the cooled cake, slice, and serve!