CHOCOLATE WAFER SANDWICH COOKIES

Steps:

• Line two large baking sheets with parchment paper; set aside. In a large bowl, whisk together flour, cocoa, baking powder, baking soda, and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and both sugars on medium speed until light and fluffy, 2 to 3 minutes. Add the egg and vanilla; beat to combine. With mixer on low speed, add flour mixture, and beat to combine, scraping down the sides of the bowl as needed.
• Turn out the dough onto a piece of plastic wrap, and divide in half. With floured hands, shape each piece into a flattened rectangle, wrap with plastic, and refrigerate until firm, about 30 minutes.
• Place one rectangle of dough on a lightly floured work surface. Roll out dough to a scant 1/8-inch thickness, stopping every so often to release the dough by running an offset spatula underneath. You should end up with a rectangle that's about 14 by 11 inches. Transfer dough to a prepared baking sheet, and freeze until very firm, about 30 minutes. Repeat with remaining dough.
• Preheat the oven to 350°F. Place one rectangle of dough on a clean work surface. Working quickly, cut out rounds using a 2-inch cookie cutter. (If the dough begins to soften too much, return to the freezer for a few minutes.) Using a wide metal spatula, transfer rounds to parchment-lined baking sheets, about 1 1/2 inches apart. Gather together remaining scraps, reroll, and cut out more rounds. Freeze until firm, about 15 minutes. Repeat with the remaining rectangle of dough.
• Bake, rotating sheets halfway through, until the centers of the cookies feel firm when lightly pressed, 12 to 14 minutes. Transfer to a wire rack to cool completely.
• Using an offset spatula, spread 1 tablespoon desired filling onto the flat sides of half the cookies. Sandwich with remaining cookies, keeping the flat sides down. Unfilled cookies can be kept in an airtight container at room temperature for up to 1 week. Once filled, cookies are best eaten the day they are made, but they can be kept in an airtight container in the refrigerator for up to 3 days.
• In the bowl of an electric mixer fitted with the paddle attachment, combine all ingredients. Beat on medium-high speed until fluffy and light, 3 to 4 minutes. Use immediately or refrigerate in an airtight container for up to 3 days. Let soften at room temperature before using.
• Melt chocolates in a heatproof bowl set over (but not touching) simmering water, stirring occasionally. Add condensed milk, butter, and extract; stir to combine. Continue to stir over simmering water until all ingredients are smooth, 2 minutes. Remove from heat, and set aside to cool slightly.