Preheat oven to 350 degrees and grease a 9 x 13 baking pan.
In a small bowl, combine flour, baking powder, pumpkin pie spice, and salt; set aside
In a large bowl, cream together the butter and brown sugar. Add the eggs and vanilla extract, and beat until well combined.
Add the flour mixture and stir just until combined.Fold in the chopped cranberries and white chocolate chips.
Spread the batter evenly into the prepared baking dish.
Bake for 20–30 minutes until the bars are lightly brown on top and a tooth- pick comes out clean. Allow to cool completely.
Frosting
Combine the frosting ingredients in a large bowl and beat together until smooth.
Spread the frosting over the cooled bars.
Sprinkle the chopped cranberries over the frosted bars and drizzle with ¼ cup melted white chocolate.
Chill for at least 30 minutes and cut into bars.
Serve cold or at room temperature. Store in the refrigerator.