Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch baking pan.
In a medium bowl, whisk together flour, baking powder, and salt.
In a large bowl, cream together brown sugar, granulated sugar, and melted butter. Beat in egg and vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Stir in cranberries and white chocolate chips.
Pour batter into prepared pan and spread evenly.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached.
Let cool completely before frosting.
For the frosting, in a medium bowl, beat together cream cheese and butter until smooth. Gradually add powdered sugar, beating until smooth. Stir in milk and vanilla extract.
Frost cooled bars and cut into squares. Garnish with additional cranberries and white chocolate chips, if desired.