Heat oven to 350ºF.

In a bowl, combine flour, sugar, baking powder, cinnamon, baking soda, and ginger. Stir in pumpkin, butter, and eggs; mix well. Stir in cranberries (if using).

Spread batter into ungreased 15x10x1-inch jelly-roll pan. Bake for 20 to 25 minutes, or until toothpick inserted into the center comes out clean. Cool completely.

In a 1-quart saucepan over medium heat,  melt butter, stirring constantly and watching closely, until butter just starts to turn golden brown (3 to 5 minutes). Immediately remove from heat. Pour into a bowl and cool for 5 minutes. Add sugar and vanilla to cooled browned butter and mix well. Stir in enough milk for desired frosting consistency.

Spread frosting over cooled bars. Cut into bars.