In a food processor, pulse hazelnuts with flour until finely ground. Add sugar, butter, extract, and eggs. Process until smooth. Transfer filling to a bowl, cover, and refrigerate for 3 hours.
Preheat oven to 350 degrees. In a small bowl, combine crumbled chocolate sandwich cookies with melted butter. In a 9-inch tart pan with a removable bottom, spread the mixture evenly on the bottom and up the sides. Bake for 10 minutes.
Allow the crust to cool slightly then lightly spread hazelnut spread over the base of chocolate crust. Dollop the refrigerated hazelnut filling all over and spread it carefully with an offset spatula. Bake until golden and toothpick comes out clean, about 45 minutes. Allow to cool completely.