Combine flours, powder, soda, salt, pumpkin pie spice and Xanthan together in a medium size mixing bowl. In a separate bowl, whisk xylitol and all wet ingredients until combined.
Pour wet mixture into the dry ingredients and fold together with a spatula.
When combined pour contents into a 9x13 greased pan or collared. Bake at 350ºF for 30 minutes.
Beat in stand mixer for 5- 8 minutes, scraping down sides when necessary.
Spread the top of the cooled pumpkin bar with entire contents of frosting recipe. Smooth and texture top. Using a knife with hot water, cut bars 2x6 into 12 rectangle pieces.