Preheat oven to 350° F / 176° C. Line a half-sheet rimmed tray with parchment, and set aside until ready to use.

In a large bowl, whisk together 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 or the bowl of a stand mixer, whisk together melted butter, brown sugar, and granulated sugar.

Mix in the pumpkin purée, followed by the eggs.

With a flexible rubber spatula, fold the dry ingredients into the batter. Mix until just combined.

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

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 light and fluffy.

Add the confectioner’s sugar mixture to the butter, 1/2 cup at a time, mixing well after each addition.

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

Stir in the chopped candied ginger.

Transfer to a piping bag.

Assembly:

Divide the cakes into pairs.

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

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

Store leftover whoopie pies in an airtight container at room temperature for up to three days. If desired, whoopie pies can also be wrapped individually in clingfilm/plastic wrap.