Place oven rack in middle position and Preheat oven to 350 degrees (F).

STEP 1
Toast hazelnuts in a shallow baking pan until fragrant and skins begin to loosen, about 6 minutes. Rub nuts in a kitchen towel to remove any loose skins (some skins may not come off), then cool to room temperature. Pulse nuts and 1/4 cup brown sugar in a food processor until nuts are finely ground.

STEP 2
Whisk flour, baking powder, salt, and cinnamon together in a small bowl. Beat together butter and remaining 1/4 cup brown sugar in a large bowl with a mixer at medium-high speed until pale and fluffy, about 3 minutes. Add nut mixture and beat until combined well. Beat in egg and vanilla. Reduce speed to low; add flour mixture, mixing until just combined.

STEP 3
With floured hands, form dough into 2 balls and flatten each into a 5" disk. Chill disks, wrapped in plastic wrap, at least 2 hours.

STEP 4
Place oven racks in upper and lower thirds of oven. Roll out 1 disk of dough into an 11" round (1/8" thick) between 2 sheets of wax paper (keep remaining dough chilled). If dough becomes too soft to roll out, re-wrap in plastic and chill until firm. Cut out dough with 3" - 4" cookie cutter and transfer to 2 ungreased baking sheets, placing 1" apart. Using mini cutters 1 1/2", cut out centers from half of the cookies, reserving centers and re-rolling along with scraps.

STEP 5
Bake cookies until edges are golden, 10 - 15 minutes, then transfer with a metal spatula to racks to cool completely.

STEP 6
Spread about 1 teaspoon jam or frosting on flat side of 1 solid cookie and sandwich jam with flat side of 1 windowed cookie. Sandwich remaining cookies in same manner. Cooks note: Cookies keep, layered between sheets of wax paper or parchment, chilled in an airtight container 2 weeks. Makes about 2 dozen cookies.