Combine flour, cocoa powder, baking powder and salt in a medium bowl.  

In a large separate bowl of a stand mixer, cream the butter and sugar together until light and fluffy.  Add the egg and vanilla and mix well, scraping down the sides of the bowl.  

Gradually add the flour/cocoa powder mixture on low speed until combined.

Divide dough in half and shape each half into a log about 9 inches long and 1 ½ inches in diameter.  Wrap tightly in plastic wrap and refrigerate 1 hour or until firm.

Heat oven to 350 degrees and line a baking sheet with parchment paper.  Remove dough from the refrigerator and slice into ¼-inch thick slices with a sharp knife (I find a non-serrated knife works best).  Place on the baking sheet and bake for 10 minutes, or until cookies are set.  Transfer baked cookies to wire racks to cool completely.

Assemble cookies by spreading about 1-2 teaspoons of nutella on the underside of one cookie, then topping it with another cookie, creating a cookie sandwich.  Refrigerate for 30 minutes to allow the nutella to firm up.