Preheat the oven to 350 degrees.

In a medium-sized bowl, whisk the flour, baking powder, baking soda and salt together and set aside.

In the bowl of a stand mixer, fitted with the paddle attachment, combine the shortening, butter, and sugar, and mix on medium-low speed until slightly fluffy, scraping down the bowl with a rubber spatula, as needed. Add the yolks and vanilla and mix again on medium low until just incorporated.

Place the cocoa and espresso powders in a small bowl and add the boiling water. Whisk until combined and add this chocolate mixture to the stand mixer bowl and mix on medium low to combine, scraping down the bowl with a rubber spatula as needed.

Add the dry mixture in three additions to the stand mixer bowl, alternating with the buttermilk in two additions, and ending with the dry. Take the bowl off the mixer when there is still unincorporated flour in the dough and finish mixing by hand with a rubber spatula. Let the dough rest on the counter, lightly covered with saran wrap, for at least 30 minutes.

Line two baking sheets with parchment paper and drop tablespoons of dough about 1-inch apart on the sheets. Bake for 8 to 10 minutes, rotating the pans halfway through the baking time until the cookies are dry to the touch and bounce back when lightly pressed. let cool completely before filling with tablespoons of the above buttercream frosting, or one of your own choosing.

Serve immediately, or cover with saran wrap and refrigerate for up to 24 hours. Bring to room temp before serving. Whoopie pies are best consumed within 2 to 3 days.