Ingredients
2 1/2 cups unbleached all-purpose flour
2 teaspoons baking powder
3/4 teaspoon kosher salt
1 teaspoon ground cinnamon
1/4 teaspoon ground allspice
1 cup whole milk
1 teaspoon pure vanilla extract
5 tablespoons unsalted butter, room temperature
1/2 cup granulated sugar
1/2 cup packed light-brown sugar
1 large egg, room temperature
1/3 cup granulated sugar
Pinch of kosher salt
3 tablespoons heavy cream
2 teaspoons unsalted butter
1 1/4 cups confectioners' sugar
1/4 teaspoon pure vanilla extract
7 tablespoons unsalted butter, room temperature
2 tablespoons coarse sanding sugar
3/4 teaspoon pink rock salt, such as Himalayan
Preparation
Cookies: Preheat oven to 375 degrees. Whisk together flour, baking powder, kosher salt, cinnamon, and allspice. In another bowl, combine milk and vanilla.
In a large bowl, beat butter with granulated and brown sugar on high speed until smooth, about 3 minutes. Add egg; beat until pale, about 2 minutes. Reduce speed to low and beat in flour mixture in two batches, alternating with milk mixture. Increase speed to medium-high and beat until just combined, about 10 seconds. Transfer mixture to a pastry bag fitted with a 5/8-inch round tip (such as Ateco #808).
Pipe 2 1/2-inch-diameter mounds onto 3 parchment-lined baking sheets, spaced about 3 inches apart (no more than 8 per sheet). Bake, rotating sheets halfway through, until cookies spring back when lightly pressed, 12 to 14 minutes. Transfer sheets to wire racks; let cool completely.
Caramel: In a small high-sided saucepan over medium heat, combine sugar, salt, and 2 tablespoons water, stirring until sugar is dissolved. Continue to cook, without stirring, and washing down sides of pan with a wet pastry brush to prevent crystals from forming, until caramel is a deep amber, 8 to 10 minutes. Remove from heat.
Carefully whisk in cream (it will spatter), then butter, until combined. Let cool completely, about 45 minutes.
Buttercream: Meanwhile, in a large bowl, beat butter on medium-high speed until pale and creamy, about 2 minutes. Reduce speed to medium; add sugar, 1/4 cup at a time, beating after each addition, until combined, 1 to 2 minutes. Beat in vanilla.
Beat caramel into buttercream on medium speed, scraping down sides of bowl as necessary, until smooth, about 3 minutes. Makes 2 cups.
Put buttercream in a pastry bag fitted with the 5/8-inch round tip. Pipe 2 tablespoons onto flat sides of half of cookies, then sandwich with remaining cookies. Refrigerate until firm, about 30 minutes.
In a small bowl, stir together rock salt and sanding sugar. Roll edges of sandwiches in sugar-salt mixture. (Whoopie pies can be refrigerated in airtight containers up to 3 days.)
