In a food processor, pulse together the flour, confectioners' sugar, ground hazelnuts, baking powder and salt until completely combined. Add the cold butter all at once, and continue to pulse until it comes together.
Place the dough onto a sheet of plastic wrap and flatten to about 1 inch, then wrap well and refrigerate until firm, about 30 minutes.
Preheat the oven to 325 degrees F. Line 2 cookie sheets with parchment paper or silicone baking mats.
Pinch off teaspoon-sized pieces of dough and roll into small balls, then transfer to the prepared cookie sheets, spacing them 2 inches apart. Bake, rotating the cookie sheets halfway through, until dry to the touch, 13 to 15 minutes. Let cool undisturbed (the cookies are fragile) on the cookie sheets on a rack.
Once cool, spread the chocolate hazelnut spread on the bottoms of the cookies and sandwich them together. Store in an airtight container for up to 1 week.