Process flour, hazelnuts, cornstarch, and salt in a food processor until hazelnuts are finely ground, about 40 seconds, stopping to stir halfway through.

Using stand mixer fitted with paddle, beat butter and sugar at medium speed until light and fluffy, about 2 minutes. Scrape down sides of bowl, add cardamom and vanilla and mix to combine.

Add flour-hazelnut mixture and beat on medium until dough resembles damp sand and holds together when pressed between fingers, 3-4 minutes. (Make sure to mix for at least 3 minutes for best results.)

Turn dough out onto a sheet of plastic wrap and firmly press together to form a 7x3-inch rectangle. Tightly wrap with plastic and refrigerate for 2 hours or up to 3 days.

Preheat oven to 325° and line baking sheet with parchment paper.

Slice chilled dough into ¼-inch thick slices and transfer to prepared baking sheet, spacing 1 inch apart. Bake 18-20 minutes, rotating pan halfway through, until edges begin to brown. Cool cookies on sheet pan 5 minutes, then transfer to wire rack to cool completely.

In a glass bowl, melt chocolate chips in microwave on high in 20 second intervals, stirring after each interval, until completely melted.

Dip cookies in chocolate, transfer to wire racks, and sprinkle with chopped hazelnuts. Once chocolate has set, store cookies in an airtight container with parchment paper between each layer.