Preheat oven to 375F and generously grease a 10x15x1 inch jelly roll pan with cooking spray. (PS - This is a good time to set out your butter/cream cheese for the frosting so it's at room temperature when you need it!)

In a large bowl, whisk together the gluten-free flour, baking soda, baking powder, salt and spices. Set aside.

In a small bowl or large measuring cup, combine the pumpkin, milk and vanilla extract, then whisk together until smooth.

In the bowl of a stand mixer, add the eggs, white sugar and brown sugar. Mix on medium high for 2-3 minutes, until fluffy and pale in color. With the mixer running on low, slowly drizzle in the oil. Mix until combined.

With the mixer running on low, add ½ of the the dry ingredient mixture. Mix until combined then add ½ of the pumpkin mixture. Repeat with the rest of the dry ingredients and pumpkin, mixing well after each addition. Scrap the bottom of the bowl to make sure everything is mixed together.

Transfer the batter to the prepared pan and spread out into an even layer using an offset spatula. Bake at 375F for about 25-30 minutes or until the center of the cake springs back when pressed lightly. Cool completely before frosting.

Using a hand or stand mixer, beat together the butter and cream cheese until combined. Add the powdered sugar and vanilla.

Beat on low then switch to high for about three minutes or until light and creamy. Scrape down the sides and bottom of the bowl then mix for another 30 seconds or so.

Smooth the frosting over the cooled pumpkin bars. Refrigerate for at least 30 minutes before slicing.