In a medium bowl whisk together the pumpkin puree, brown sugar, unrefined sugar, sourdough starter, eggs and vanilla.
Melt the butter and allow to cool about 5 minutes. Add to the pumpkin mixture and stir briefly.
Add the dry ingredients to the wet ingredients and stir well but do not over mix! The cookie batter will be very moist. That is fine.
Preheat the oven to 350. Put the dough in the fridge until the oven has reached temperature.
In the meantime, grease two cookie sheets/baking sheets or line with parchment paper or use silicone baking mats which are my personal favorite.
Using a small cookie scoop (ice cream scooper) place little mounds about 2 inches apart on the prepared baking sheet. These cookies do not spread very much.
Flatten slightly with the back of a spoon.
Place into oven and bake for 12-14 minutes. These are a soft pumpkin cookie but they should still be a bit firm and yet bouncy when you tap them with your finger. They have the texture and feel of part cake-part cookie.
Remove from the oven and allow to cool on the pan for five minutes.
Transfer to a wire rack and allow the cookies to cool to room temperature before spreading filling on them.
Make the filling by using an electric mixer and beating together everything except the powdered sugar. Add the powdered sugar and beat well. Add a dash of milk or two until it is a spreadable consistency.
Spread filling between the flat side of two cookies and sandwich them together. (Or use a piping bag for a prettier appearance). Store in the fridge for up to a week. Enjoy!