Chocolate Ganache Sandwich Cookies

Preheat oven to 400°F with rack in center of oven. Line 2 baking sheets with parchment paper.

Beat butter, powdered sugar, granulated sugar, and salt in a stand mixer with paddle attachment until pale and fluffy, scraping down sides and bottom of bowl halfway through, about 2 minutes. Beat in vanilla bean paste. Add egg whites, beating to blend and stopping to scrape down bowl as necessary. Add flour and mix on low just to incorporate.

Transfer batter to a large piping bag. For mini cookies, cut 1/2-inch off pastry bag tip, and for larger cookies, cut off 3/4-inch. Pipe 1 1/2-inch long strips for mini cookies, and 2-inch long strips for larger cookies, onto 1 prepared baking sheet, spacing 1 1/2-inches apart.

Bake cookies until bottoms are golden brown but tops remain pale, rotating sheet halfway through, 7 to 9 minutes. Transfer cookies and parchment to wire racks and let cool completely. Repeat piping and baking until batter is used up, letting baking sheets cool completely before lining with parchment paper and piping more cookies.

Place milk chocolate in a microwave-safe bowl and microwave in 15 second intervals, stirring between each interval, until fully melted, about 45 seconds. Set aside. Add butter to the bowl of a stand mixer fitted with a whisk attachment and beat briefly until smooth. Add melted chocolate to the bowl, and beat briefly to incorporate. Add the vanilla bean paste and salt, and beat until mixture is pale and fluffy, about 5 minutes. Transfer ganache to a piping bag.

Transfer half of the cookies from rack to a rimmed baking sheet, browned side up. Using sharp scissors, cut a small opening, about ⅜-inch, from tip of piping bag. Pipe filling over cookies. Top with remaining cookies, browned side down. Refrigerate cookies while preparing chocolate garnish.

Place 2/3 cup dark chocolate in a microwave safe bowl and microwave in 15 second intervals, stirring between each interval, until fully melted, about 45 seconds. Add the remaining 1/3 cup chocolate and stir until melted. Spoon into a cornet or piping bag, and snip a very small piece from the tip of the bag. Drizzle chocolate decoratively over cookies as desired; add sprinkles if desired. Refrigerate until set. Store airtight in refrigerator between layers of parchment paper. Let come to room temperature 10 minutes before serving.