Heat oven to 350° F (325° F convection) and place rack in the center of the oven.
Line your cookie sheets with parchment paper or silicone pastry mats.
In a large bowl whisk together the flour, cocoa powder, salt, and baking powder.
In the bowl of your electric mixer (or with a hand mixer), beat the butter and sugar until light and fluffy (about 3 to 4 minutes).
Add the eggs, one at a time, beating well after each addition. Add the vanilla extract and beat until combined.
Slowly add the flour mixture and beat until the dough is smooth.
Divide the dough in half and wrap each half in plastic wrap. Refrigerate for about one hour or until firm enough to roll.
Remove one half of the chilled dough from the refrigerator and, on a lightly floured surface, roll out the dough to a thickness of 1/4 inch. (Keep turning the dough as you roll, making sure the dough does not stick to the work surface.)
Cut out circles using a lightly floured cookie cutter and transfer cookies to the prepared baking sheet.
Place the baking sheets with the unbaked cookies in the refrigerator for 10 to 15 minutes to chill the dough which prevents the cookies from spreading and losing their shape while baking.
Bake the cookies for about 8-10 minutes or until they are firm around the edges.
Remove from oven and let cookies cool on baking sheet for a few minutes before transferring to a wire rack to finish cooling.
Nutella Ganache
Cook Nutella and heavy cream in heavy-bottomed sauce pan over MEDIUM heat, stirring constantly until the mixture is completely smooth (7-10 minutes). Let the ganache cool slightly (about 5 minutes). *Do NOT use HIGH heat; if you rush this, the results will be lumpy and dull in appearance.
When the cookies have cooled down but are still warm, use an offset spatula to coat the tops of the cookies with ganache. Let the ganache cool completely. Just before serving, dust with powdered sugar.