Special equipment: 2 pastry bags fitted with a 1/2-inch round tip or 2 resealable plastic bags
For the cakes: Preheat the oven to 375 degrees F. Line 2 baking sheets with parchment.
Whisk together the flour, baking powder, baking soda and salt in a medium bowl. Split the vanilla bean lengthwise with a paring knife, and scrape down the length with the back of the knife to remove the seeds. Add the seeds, granulated and brown sugars, butter and shortening to the bowl of a stand mixer fitted with the paddle attachment, and beat on medium-high speed until the mixture looks pale and fluffy, about 5 minutes. Add the eggs 1 at a time, beating after each addition, and beat for 3 minutes, scraping down the side of the bowl with a rubber spatula halfway through. Add the peppermint extract and food coloring to the milk, reduce the mixer speed to low and add the milk to the sugar-egg mixture (it may appear curdled, but that's OK). Mix until combined, then slowly add the flour mixture, continuing to mix, without overmixing, until the batter is smooth and uniform. Scrape the bowl with a rubber spatula into a pastry bag fitted with a 1/2-inch round tip or a resealable plastic bag with a corner snipped off to make a 1/2-inch opening.
Pipe rounded-tablespoon-sized mounds of the dough about 2 inches apart onto the prepared baking sheets. Bake until the cakes are set and springy when gently touched, about 10 minutes. Let the cakes cool for 5 minutes on the baking sheets, then gently transfer them to a wire rack with a spatula to cool completely.
For the filling: Beat the cream cheese and butter on medium-high speed until smooth, about 2 minutes. Slowly add the confectioners' sugar, and beat until the mixture is smooth and light. Add the bourbon, and beat for 1 minute more (the filling will be very soft).
For the mint sugar: Put the granulated sugar and mint in a food processor or mini processor, and process until the mint is finely chopped and the sugar is green.
To assemble: Pipe or spoon the filling onto the flat side of half the cakes, leaving a small border. Place another cake, flat-side down, on top of each iced cake to create sandwiches, and gently press in place. Roll the edge (with exposed filling) of each whoopie pie in mint sugar. Refrigerate them in a single layer until the filling is set, about 30 minutes. (Whoopie pies can be made the day before, refrigerated in an airtight container in a single layer and served the next day.)
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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