MINI CHOCOLATE SANDWICH COOKIES

Steps:

• Make cookies:
• Whisk together flour and salt in a bowl until combined.
• Beat together butter and sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 2 minutes in a stand mixer (preferably fitted with paddle attachment) or 4 minutes with a handheld. Add whole eggs, yolks, and chocolate, beating until combined. Reduce speed to low, then add flour mixture and mix until combined well.
• Put oven racks in upper and lower thirds of oven and preheat oven to 350°F.
• Drop 1/2 teaspoons of batter 1 inch apart on 2 ungreased baking sheets. Bake, switching position of sheets halfway through baking, until cookies are puffed up, 7 to 8 minutes total, then transfer with a metal spatula to racks to cool completely. Cool baking sheets and make more cookies with remaining dough on cooled sheets.
• Make filling:
• Whisk together eggs, confectioners sugar, butter, and chocolate in top of a double boiler or a large metal bowl set over a pot of simmering water and cook, whisking, until a thermometer registers 170°F. Remove top of double boiler (or bowl) from pot and set in a larger bowl of ice and cold water, then stir occasionally until cold. Remove from ice water and beat with cleaned beaters at medium-high speed until pale and fluffy, about 3 minutes.
• Fill cookies:
• Spread 1/2 teaspoon filling on flat side of 1 cookie, then place flat side of a second cookie over filling to form a sandwich. Fill remaining cookies in same manner.