Preheat your oven to 350°F (175°C).
Grease a 9x13-inch baking pan with cooking spray or line it with parchment paper.
In a large mixing bowl, combine the melted unsalted butter, egg, light brown sugar, and vanilla extract. Whisk until smooth.
Add the cinnamon, all-purpose flour, and salt. Mix until just combined.
Pour the batter into the prepared pan and spread evenly. Bake for 20-25 minutes or until a toothpick comes out clean.
While the bars are baking, prepare the cream cheese frosting. Beat softened cream cheese and butter together, then gradually add confectioners' sugar, vanilla extract, and salt until smooth.
Once the bars are out of the oven, let them cool for about 10 minutes, then spread a layer of cream cheese frosting over the warm bars.
Allow to cool completely before slicing into squares. Serve warm or at room temperature.