Preheat oven: Start by preheating your oven to 375°F (190°C) and lining a 9x13 inch baking pan with parchment paper for easy removal later.

Whisk dry ingredients: In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.

Cut in butter: Cut the cold, cubed butter into the dry mixture using a pastry cutter or your fingers until it resembles coarse crumbs.

Mix wet ingredients: In a separate bowl, combine the buttermilk and vanilla extract and gently add this mixture to your dry ingredients, folding until just combined.

Press the dough: Evenly press the dough into the lined baking pan ensuring a smooth surface that reaches the edges.

Prepare filling: Mix the brown sugar and ground cinnamon in a small bowl. Drizzle the melted butter over the pressed dough and sprinkle with the cinnamon-sugar mixture.

Bake: Place in the preheated oven and bake for 20-25 minutes, until the edges are golden brown. Allow to cool completely in the pan before frosting.

Make frosting: In a mixing bowl, beat the softened cream cheese and remaining butter until smooth, then gradually add the powdered sugar and milk.

Frost and cut: Once cooled, spread the creamy frosting on top and cut into squares.