Position the racks to divide the oven into thirds and preheat it to 375 degrees F. Butter or spray two standard muffin tins (do this even if the tins are nonstick).
To make the cookies: Whisk the flour, cinnamon, cardamom, baking powder and baking soda together.
Working with a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the butter, sugar and salt together on medium speed until smooth, about 2 minutes. Add the egg and beat for another 2 minutes or so, until creamy. Beat in the vanilla. Reduce the mixer speed to low, add the pumpkin puree and beat until it’s fully incorporated; don’t be discouraged when the mixture curdles — it will soon smooth out.
Turn off the mixer, scrape down the bowl and add half of the dry ingredients. Pulse to begin the mixing and then mix on low, scraping the bowl as needed, until the flour mixture is blended in. Beat in the buttermilk. Turn off the mixer, add the remaining dry ingredients, pulse and then mix on low until you have a lovely smooth batter. Switch to a flexible spatula and fold in the cranberries. Don’t be too thorough — it’s better to have an uneven mix than to break the berries and turn the batter red.
Using a medium cookie scoop, scoop out level portions of dough, or use a tablespoon to get rounded spoonfuls, and fill the muffin tins. The scoops of dough will sit upright in the center of the tins, but when baked they’ll melt evenly into the cups.
Bake the whoopie pies for 11 to 12 minutes, rotating the pans top to bottom and front to back after 6 minutes, or until they are puffed, golden brown and springy to the touch.
Transfer the pan to a rack and let the cookies rest for 5 minutes, then turn them out on the racks and allow them to cool completely.
To make the filling: Working with a stand mixer fitted with the paddle attachment, or in a medium bowl with a hand mixer, beat the butter, marshmallow crème, sugar and salt together on medium-high speed, scraping the bowl and beater(s) as needed, for about 3 minutes, until very smooth. Lower the mixer speed, add the dulce de leche and beat until thoroughly blended.
Using a spoon or a small cookie scoop, place the filling on the flat sides of half of the cookies; sandwich with the other cookies, flat sides down. The cookies can be eaten now, but the filling benefits from a 30-minute stay in the refrigerator. Just don’t eat them from the fridge — you’ll deprive yourself of the cookies’ wonderful texture.