1teaspoon(330 g) pumpkin pie spice (or 1/2 teaspoon cinnamon, 1/4 teaspoon nutmeg, 1/2 teaspoon ginger powder, and a pinch of cloves)
1-3tablespoons(15-45 mL) non-dairy milk of choice (as needed)
Preheat your oven to 350F. Line two baking sheets with parchment.
Whisk the flour, cocoa, sugar, salt, baking powder and baking soda together in a large mixing bowl. In a separate bowl, whisk together the water, vinegar, vanilla and oil. Add the wet ingredients to the dry ingredients and whisk until batter is glossy and smooth. If the mixture isn't yet glossy, add an extra few tablespoons of water as needed (up to 1/4 cup).
Use a 1.5-2 tablespoon cookie scoop (or a spoon) to drop the cake batter in rounds onto your cookie sheet. You can make larger or smaller whoopie pies by adjusting how big your mini-cakes are (mine were about 2 inches across before baking and a little wider after). Bake the mini-cakes for 8-10 minutes, or until set but not browning. Cool the cakes on a cooling rack for an hour or two, or until fully cooled.
To prepare the buttercream, place the butter and vanilla into the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed for 1-2 minutes, or until the butter is soft and fluffy. Mix the sugar, pumpkin pie spice, and turmeric, then add this mixture in 2-3 installments, beating on medium speed as you go. If the vegan butter you use isn't salted, add a pinch of salt, too.
Add 1-3 tablespoons of non-dairy milk to the buttercream and continue beating until it's still rich and firm, but soft enough to pipe. Add the milk slowly and use your judgment: you can add more milk if it's too firm or a little more sugar if it accidentally gets runny.
When the buttercream is ready, place it in a piping bag and pipe out two tablespoons onto the bottom of one of your mini-cakes in a circle pattern. Place another mini-cake on top and press gently to make the whoopie pie. Continue with the remaining buttercream and cakes; you should get about 18-24, depending on the size of your cakes! Enjoy.
*In place of all-purpose flour, you can use light spelt, whole wheat pastry, or gluten-free all-purpose flour.