Preheat your oven to 375°F (190°C) and line an 8×8 inch baking pan with parchment paper.
In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.
Cut the cold, cubed butter into the flour mixture using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Stir in the buttermilk and vanilla extract until a soft dough forms; do not overmix.
Press two-thirds of the dough into the bottom of the prepared pan.
In a small bowl, combine the brown sugar and cinnamon; sprinkle this mixture evenly over the dough base.
Drop remaining dough in small bits over the cinnamon layer.
Bake for 20-25 minutes or until golden brown.
While the bars cool, beat the cream cheese, powdered sugar, and milk together until smooth.
Drizzle the icing over the cooled bars before cutting into squares.