Into a medium bowl, whisk together flour, black cocoa powder and baking powder. Set aside.
In a large bowl, using an electric mixer fitted with the paddle attachment on medium speed, beat butter and sugar for about 3 minutes, or until light and fluffy. Beat in the egg and vanilla until well combined. Scrape the sides of the bowl with a rubber spatula.
Turn the mixer speed to low and carefully add the flour and cocoa powder mixture a little at a time, occasionally stopping to scrape the sides of the bowl. Once all the flour has been incorporated, the dough should form a ball around the mixing attachment and feel soft but not sticky.
Wrap the dough ball in a piece of plastic wrap and press down to form a 1-inch-thick disk. Refrigerate for at least 10 minutes before rolling or store for up to 7 days tightly wrapped.
Preheat the oven to 375°F.
Roll out the dough on a lightly floured flat work surface to about ¼-inch thickness, using additional flour as necessary to prevent sticking. Use cookie cutters to create desired shapes, and carefully transfer with a cookie spatula to a nonstick baking sheet, placing the cookies about ¾ inch apart.
Bake one sheet at a time in the middle of the oven until puffy, about 7-8 minutes. Allow the cookies to rest for 2 minutes on the baking sheet before transferring to a wire rack to cool completely.
Repeat with the gathered scraps and remaining disk until all the dough has been used.
Once the cookies are cooled, fill each sandwich with cookies and cream buttercream.
Enjoy!