Preheat your oven to 375°F (190°C) and line a 9x13 inch baking pan with parchment paper.

In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.

Cut in the cold, cubed unsalted butter into the dry mixture until the texture resembles coarse crumbs.

In a separate bowl, combine the buttermilk and vanilla extract. Pour this into the dry ingredients and mix gently.

Press the dough evenly into the bottom of your prepared baking pan.

In a small bowl, combine the brown sugar and ground cinnamon. Drizzle the melted butter over the pressed dough and sprinkle the cinnamon-sugar mixture on top.

Bake for 20-25 minutes, until golden brown and a toothpick comes out clean.

Cool the bars in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the frosting by beating the softened cream cheese and butter, then gradually add in powdered sugar and milk until creamy.

Spread the creamy frosting over the cooled bars, cut into pieces, and serve.