Preheat oven to 350°F. Butter or spray an 8x12 jelly roll pan and set aside.
In a large bowl, whisk melted butter and brown sugar. Add egg and vanilla extract, whisk until combined.
In a separate bowl, combine flour, baking powder, and salt. Add flour mixture to butter mixture and stir until the ingredients are combined. Fold in 1/2 cup of dried cranberries and 1/2 cup of white chocolate chips. Spread the batter in an even layer in the prepared pan (the batter will be thick).
Bake the 8x12 pan for around 15 minutes. The bars are done when they have puffed slightly, are light brown and a toothpick inserted into the center comes out clean. They will appear a little underdone but they will continue to cook as they cool. Be careful not to over bake them. Cool completely on a wire rack before frosting.
FOR THE FROSTING:
Using a hand mixer or stand mixer, beat the cream cheese and butter until smooth. Add the powdered sugar and salt and beat until combined, scraping the sides of the bowl as needed. Add in half of the melted white chocolate and beat to combine. Add a splash of milk, if necessary, to thin the frosting. Spread the bars with the frosting and scatter the dried cranberries on top. Drizzle the remaining melted white chocolate over the top.
Store the bars tightly covered in the refrigerator.