- Preheat oven to 350 degrees. In a food processor, grate nuts and ¼ cup brown sugar together.
- Whisk together flour, baking powder, and salt in a small bowl.
- Beat together butter and remaining brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy. Add nut mixture and beat until combined. Beat in egg and vanilla.
- Add flour mixture in increments until combined. Do not overbeat.
- With floured hands, form dough into two balls and flatten each into a 5-inch disk. Chill disks, wrapped in plastic wrap, until firm, at least 2 hours.Roll dough out between two sheets of wax paper into ⅛-inch thickness. Cut out cookies (half with ½-inch “windows” and half in full 2-inch circles) and place on ungreased cookie sheet. Bake for 10 minutes or until golden. Cool.
- Spread jam on flat side of one solid cookie and sandwich jam with flat side of one windowed cookie. Sandwich remaining cookies in same manner. Sift confectioner’s sugar over cookies and serve.