Sandbakkels means "sand tarts" because the cookies (which, flipped over, can double as tiny shells to hold cream and fruit) have such a fine, crisp texture. Find them in Shauna Sever's book Midwest Made: Big, Bold Baking from the Heartland.
In a large bowl, beat butter with an electric mixer on medium-high speed about 30 seconds. Add sugars; beat until light and fluffy, about 3 minutes. Add egg and vanilla; beat on medium speed until combined. Add flour and salt; beat on low speed until well mixed, scraping sides of bowl as needed. Cover and refrigerate until dough is firm, about 2 hours.
Position oven racks in upper and lower thirds of oven. Preheat oven to 375°. Lightly coat sandbakkels tins or other decorative mini (2-inch) tart tins with cooking spray. Pinch off walnut-size pieces of dough and place in prepared tins, pressing across bottom and a bit up the sides of the tins. (Chill remaining dough between batches.) Place filled tins 2 inches apart on baking sheets. Bake until golden, 8 to 10 minutes, rotating sheets from top to bottom and front to back about halfway through.
Line a work surface with newspaper or brown paper bags. When cookies come out of oven, transfer tins to paper and let cool 2 or 3 minutes. Invert tins onto paper and tap bottom of tins a few times to release cookies from tins. Let cookies cool completely on paper. Cool tins before making the next batch.
Eat cookies straight up or fill with custard, whipped cream and/or berries. Store unfilled cookies in an airtight container between sheets of parchment paper up to 1 week.
Look for sets of fluted tins in thrift stores or online. If you don't have enough 36, you'll have to make these cookies in batches.