Make sure your butter is out of the refrigerator so it can come to room temp.
Add the sugar, flour, salt, and spices to a 1½-2 quart saucepan. Turn the heat on medium low and whisk to combine everything.
Slowly pour in about ¼ cup of milk and whisk until smooth. Continue adding in the milk in ¼-½ cup increments and whisk after each addition until smooth. By slowly adding the milk, you will reduce the likelihood of clumps forming. It's also important to keep the heat at medium low or medium, any hotter and the flour will thicken too quickly and you may have lumps.
Once all the milk is added in and the mixture is smooth, continue heating over medium low heat for about 5-7 minutes, whisking constantly. The mixture will begin to thicken. Once it is about the thickness of a pudding (before chilling) or gravy, remove from the heat.
You can chill the flour/sugar mixture (pudding) in the same pot or transfer it to a bowl. Make sure to cover it with plastic wrap and press the plastic wrap onto the surface to avoid a skin from forming. Refrigerate for 1-2 hours. The temperature should be about room temperature when mixing with the room temperature butter. You can speed up the chilling process by using a shallow container with the pudding mixture spread out on the bottom.
While the pudding is chilling, you can make the pumpkin cookies. Also, make sure your butter is at room temperature before proceeding to the next step.
Place the room temperature butter into the bowl of a stand mixer with the whisk attachment or use a large bowl with a hand mixer. Whip the butter on high speed until it lightens in color and becomes fluffy. This takes about 2-3 minutes.
With the mixer running on medium speed, add in about ¼-½ of the room temperature pudding mixture and whisk until combined. Repeat until all of the pudding mixture is combined. The buttercream should be light and fluffy. You can switch over to the paddle attachment and beat for another minute or two if you need to smooth out the buttercream (this is not necessary though.)
Pumpkin Cookies
Preheat the oven to 325°F/160°C. Melt the butter in the microwave or on the stove.
You can use a stand mixer, a handheld electric mixer or mix the cookie batter by hand. If you are using a bowl and spatula to combine everything, lightly whip the eggs first to make it easier for them to evenly combine with the other ingredients.
In the bowl of a stand mixer or large mixing bowl, add 2 cups of the flour, spices, baking powder, and salt. Whisk together to combine, making sure to break up any clumps of spices so they distribute through the flour.
Add the melted butter, eggs, sugar, pumpkin puree, and vanilla extract to the dry ingredients and mix until combined. The batter should be slightly sticky to the touch, but thick enough to hold it's shape. For a sticky and soft cookie, 2 cups of flour is usually enough, but if you want the cookies a little firmer, add in the remaining ¼ cup of flour and mix to combine. *See notes below
Use between 1 and 3 Tablespoons of batter for each cookie depending on how big you want the whoopie pies. A 1 Tablespoon scoop will give you a small whoopie pie that is about 2-3 bites. A 3 Tablespoon scoop is a large size whoopie pie, probably too much for one person to eat, but usually what you see in shops that sell whoopie pies.
Scoop the batter onto the prepared cooking sheet, leaving about an inch between each cookie to allow for spreading. Bake in a preheated oven on 325°F/160°F for 15-17 minutes. The tops should be set completely and the cookies should release easily from the pan.
Remove the cookies from the tray with a spatula and let them cool completely on a cooling rack. This is very important because if you try to fill the cookies while they are warm, the filling will melt and become a greasy mess. You can pop them in the fridge to cool faster, but leave them in a single layer until they are completely cool or they can stick to each other.
Assembling the Pumpkin Whoopie Pies
Once the Spiced Buttercream is mixed up and the cookies have cooled completely, it's time to assemble the whoopie pies.
Each whoopie pie needs two cookies that are about the same size. Usually the amount of filling is equal to the number of Tablespoons you used for the cookie size, but you can use as much or as little as you like.
Place a scoop of the spiced buttercream onto the flat bottom of one pumpkin cookie. Place a second cookie on top with the flat bottom facing the buttercream. Repeat until all the whoopie pies are assembled.
Storage of Whoopie PIes
Personally, I like to refrigerate my whoopie pies in a single layer or with parchment paper between each layer because I like the buttercream to firm up a bit, but you can also store them at room temperature for up to 6 hours.
Room Temp: Up to 6 hours
Refrigerator: 3-5 days when stored in a covered container
Frozen: 3-6 months when frozen and stored correctly. Freeze in a single layer until completely frozen. Wrap each whoopie pie in plastic wrap and store in the freezer. You can put several wrapped whoopie pies in a freezer bag for extra protection.
Cookie ConsistencyYou can use between 2 and 2¼ cups of flour depending on how you want your cookies to be. 2 cups of flour will give you a soft, moist, but slightly tacky cookie. This means that some of the cookie may stick to your fingers when you are eating it. For a slightly firmer cookie that won't stick to your fingers, add the additional ¼ cup of flour to the mix. YieldMedium Size: This recipe makes about 12 medium size whoopie pies. Medium size uses about 2 Tablespoons of cookie batter per cookie and about 2 Tablespoons of filling per whoopie pie. You will have some leftover buttercream, so you can increase the filling to about 2½ Tablespoons if desired. Mini Whoopie pies: This recipe will make about 24 mini whoopies when using 1 Tablespoon of batter per cookie and about 1-1½ Tablespoons of buttercream per whoopie pie. Mini Whoopie Pies are about 2-3 bites each, making them great for a dessert tray. Large Whoopie Pies: This recipe will make about 8 large whoopie pies when using about 3 Tablespoons of batter per cookie and 3-4 Tablespoons of buttercream per whoopie pie.