Preheat oven to 350 degrees. Lightly grease a 9x13 inch pan and lay a piece of parchment (or wax paper or foil) paper across the pan, so that it extends beyond the pan (the paper is an optional step, but it will make it easier to get the bars out later).
To make Snickerdoodle layer: Sift together flour, baking powder and salt and set aside. In large bowl, beat together butter, sugar, egg and vanilla until smooth.Stir in the flour mixture until well blended. Spread evenly in prepared pan.
To make Pumpkin layer: In a mixer bowl, mix together all ingredients until well combined. This layer will be less thick and more pourable. Pour over the snickerdoodle layer, smoothing out the top.
To make topping: Combine white sugar and cinnamon in a little bowl. Evenly sprinkle cinnamon sugar mixture over the top of the batter.
Bake for 33-40 minutes or until a toothpick inserted into the center of the pan comes out clean. Let the bars cool completely. They will deflate a bit and remain a bit pie-like on the top layer. The bars that are closer to the edges of the pan will be more firm, but are still equally as good.
To make drizzle: After the bars are completely cool, place the chopped white chocolate into a bowl or zip-lock bag and melt on low power. When it's completely melted, add the pumpkin pie spice and mix. Use a spoon or cut a small corner off the bag and drizzle the melted chocolate over the top of the bars and let it cool and harden.
Use the parchment paper to lift the bars out of the pan. Place on a cutting board and cut into bars.