Preheat oven to 350° F (176° C). Line a half-sheet rimmed tray with parchment or a silicone baking mat.

In a large bowl, whisk flour, baking powder, baking soda, ground cinnamon, ground ginger, ground cardamom, ground nutmeg, ground cloves, and kosher salt. Set aside.

In a separate mixing bowl, whisk melted butter, brown sugar, and granulated sugar.

With a hand mixer, mix in the pumpkin purée, followed by the eggs.

With a flexible rubber spatula, fold the dry ingredients into the batter. Mix on low speed until just combined, with no dry pockets remaining.

Transfer batter to a piping bag fitted with a 3/4-inch round tip.

For smoother cakes, allow batter to rest in the piping bag for 10-15 minutes.

Pipe 1 1/2-inch – 2-inch rounds onto prepared tray, allowing space for the cakes to spread as they bake.*

Bake in preheated oven for 12-13 minutes, or until cakes are nicely risen and set around the edges, and spring back lightly when touched.

Repeat with remaining batter.

Allow whoopie pies to cool completely on tray. Meanwhile, prepare the buttercream.

Chocolate Buttercream:

In a large bowl, whisk confectioner’s sugar, cocoa powder, and salt. Set aside.

In a separate bowl or the bowl of a stand mixer, whip the softened butter until smooth and creamy.

Add the dry ingredients 1/2 cup at a time, mixing well after each addition.

Mix in the whole milk and ginger oil, if using. If necessary, add a bit more milk or confectioner’s sugar to adjust the consistency. The buttercream should be thick and creamy rather than runny or watery, but still thin enough to pipe.

Stir in the chopped candied ginger.

With a flexible rubber spatula, scoop chocolate buttercream into a piping bag.

Assembly:

Divide the cakes into pairs.

Pipe a generous dollop of buttercream on the flat side of half of the cakes.

Top with the remaining cakes, using your hands to gently sandwich the 2 halves together.

Store leftover whoopie pies in an airtight container at room temperature for up to 3 days.