Preheat your oven to 350°F (175°C) and line an 8×8 inch square baking pan with parchment paper.
In a large bowl, whisk together the melted butter and brown sugar until smooth.
Add the egg and vanilla extract, whisking until well combined.
Fold in the flour, baking powder, salt, ginger, and cinnamon until just incorporated.
Stir in the white chocolate chips and dried cranberries.
Spread the batter evenly into the prepared baking pan.
Bake for 20 to 25 minutes or until the edges are golden brown and a toothpick comes out mostly clean.
Allow the bars to cool completely in the pan on a wire rack.
While cooling, beat the cream cheese and powdered sugar together until smooth, adding milk to reach your desired spreading consistency.
Once cool, spread the frosting over the bars.
Melt the remaining white chocolate and drizzle it over the top for a final flourish.
Allow the frosting to set before cutting into squares.