Preheat the oven to 275 degrees F. Line 2 large baking sheets with parchment paper.
the confectioners' sugar and walnuts in a food processor
until finely ground. Add the cocoa powder and pulse until well blended.
With an electric mixer
on medium speed, beat the cream of tartar
whites and salt until foamy. Beat in the granulated sugar
, 2 tablespoons at a time, on high speed until the whites stand in stiff glossy peaks, about 2 minutes. Beat in the vanilla. With a rubber spatula
, gradually fold in the walnut mixture and chocolate chips. The batter will deflate slightly.
Transfer the batter
to a large pastry bag fitted with a 1/2-inch plain tip. Pipe 1 1/2-inch mounds about 1-inch apart onto the baking sheets. Bake, rotating the sheets halfway through baking time until the cookies are crisp
, about 30 minutes. Slide the parchment onto wire racks to cool 10 minutes. Gently peel
the parchment off the cookies. Transfer to wire racks to cool completely.
Spread 1/2 teaspoon of the chocolate hazelnut spread on the flat side of half of the cooled cookies. Top with the remaining cookies, flat-side down, pressing together gently to form sandwiches. Store the cookies
between layers of wax paper in an airtight container for up to 2 days.
Cook's Note: Be sure to use mini chocolate morsels as regular morsels are too big to fit through the piping tip. If these get stuck, dislodge them by swirling a toothpick in the piping tip. For a smooth, round cookie, pipe them in one vertical motion without making a circular movement with the pastry bag
. Flatten any peaks in the piped cookie with a moistened finger before baking.