Whisk together flour, baking powder, and salt in a small bowl. In a large bowl, beat together butter and sugar with an electric mixer until light and fluffy. Add eggs and vanilla; beat until combined. With mixer on low speed, alternately beat in chocolate and flour mixture. Mix until just combined (don't overmix!).
Use small cookie scoop to drop heaping teaspoons of batter onto baking sheets, about two-inches apart. Bake until toothpick inserted into center of cookies comes out clean, about 8-12 minutes. Cool completely on wire racks.
Mint Filling
Place butter in a small mixing bowl. Using an electric mixer, beat butter until fluffy. Add sugar and extract; continue to beat until smooth.
Chocolate Filling
Place chocolate and butter in double boiler; heat until smooth, stirring often.
Assembly
Use Decorator Bottle to pipe mint filling onto half of the brownie cookies. (If you don't want to pipe the filling onto the cookies, you can spread it on with a butterknife or spatula instead.) Spread chocolate filling onto remaining cookies. Once chocolate has cooled to room temperature, carefully press together mint and chocolate cookies. (Don't put too much pressure on the tops of the cookies--they are delicate! Instead, wiggle them back and forth, kind of like how you take apart an Oreo, but in reverse.)
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