Combine hazelnuts and ½ cup of flour in the bowl of a food processor. Process until the nuts are finely ground, then add the remaining 1½ cups of flour and the baking powder. Process with quick on/off pulses to combine.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and cream cheese on medium speed until combined, about 30 seconds. Add sugar and continue beating until light and fluffy. Add egg and vanilla and beat until smooth. Add hazelnut-flour mixture and beat on low speed until dough is combined and smooth.

Divide dough into four portions. Shape each portion into an 8-inch-long roll. Wrap in plastic wrap or wax paper and refrigerate for at least one hour or until dough is firm enough to slice.

Heat oven to 350º and line a baking sheet with parchment paper or nonstick baking mat. Using a sharp knife, cut each roll into ½-inch thick slices (16 cookies per roll). Place 1 inch apart on prepared baking sheet.

Bake for 9-10 minutes on the center rack, until the cookies are just barely golden around the edges. Transfer to a wire rack to cool completely.

Once cooled, spread about 1 teaspoon of Nutella on the bottom of half the cookies. Top with remaining cookies, bottom side down, and press gently together.

To store, place unassembled cookies in an airtight container up to 3 days or freeze up to 3 months. Thaw, if frozen, and assemble as needed.