Preheat your oven to 375F. Line a cookie sheet, also called an 8X13" half sheet pan (the kind with sides!), with parchment paper and spray generously with cooking spray to prevent sticking. Set aside.
Combine the sourdough starter, pumpkin puree, eggs, and sugar. Whisk well. In a separate bowl, whisk together the flour, baking powder, pumpkin pie spice, and salt.
Gradually add the dry ingredients to the wet mixture, folding gently until just combined. Over mixing the batter will cause the pumpkin roll to be flat and dense.
Spread the batter evenly into the prepared sheet pan. Bake 15 minutes or until done.
Once the cake has finished baking, immediately invert the cake onto a powdered sugar dusted tea towel. Loosely roll up the cake and towel from the short side and let cool completely.
While the cake is cooling, prepare the filling by beating all ingredients with a stand mixer until smooth and creamy.
Unroll the cake and spread the filling to within ½ inch of the edges. Roll the cake up again, without the towel this time, starting at the short side.
Wrap the pumpkin roll in plastic wrap and place it in the refrigerator. Chill for at least 2 hours to give the flavors time to blend.
To serve, unwrap the pumpkin roll and sprinkle it with powdered sugar. Slice in 1-inch slices. We like to dust it with more powdered sugar just before serving!