Put the butter in the work bowl of a food processor* fitted with the metal blade and process, scraping down the sides of the bowl as needed, until the butter is smooth. Add the sugar and process and scrape until thoroughly blended into the butter. Add the egg and continue to process, scraping the bowl as needed, until the mixture is smooth and satiny. Add the flour all at once, then pulse 10-15 times, until the dough forms clumps and curds and looks like streusel.

Turn the dough out onto a work surface and gather it into a ball. Divide the ball in half, shape each half into a disk, and wrap the disks in plastic. If you have the time, chill the disks until they are firm, about 4 hours. If you’re in a hurry, you can roll the dough out immediately; it will be a little stickier, but fine. (The dough can be wrapped airtight and refrigerated for up to 4 days or frozen for up to 1 month.) (SoupAddict's note: I chilled the disks for about an hour. They're going to warm up quickly as you work them anyway, so, a 4-hour chill will be quickly undone. The keys to avoiding sticky dough: 1) lay down a very light dusting of flour on the work surface before placing the dough; 2) keep your rolling pin lightly floured throughout the entire process; and, 3) be patient - roll in only one direction at a time, starting from the center outward.)

Position the racks to divide the oven into thirds and preheat the oven to 350°F (180°C). Line two baking sheets with parchment paper.

Working with one disk at a time (keeping the other in the fridge), roll the dough out on a lightly floured surface until it is between 1/8 and 1/4 inch thick. (SoupAddict's note: I have a silpin rolling pin - which I love - and purchased those measured-thickness rubberbands that go on the ends that help you roll out dough to a specific, even thickness. It worked perfectly.) Using a 1-1/2-inch round cookie cutter (SoupAddict's note: I used a fluted cutter to create the scalloped edges), cut out as many cookies as you can and place them on the lined sheets, leaving about 1 inch space between them. (Gather the scraps into a disk and stick them in freezer for a few minutes. Then roll, cut, and bake them as well.)

Bake the cookies for 8 to 10 minutes, or until they are set but pale. (If some of the cookies are thinner than the others, the thin ones may brown around the edges.) Transfer the cookies to cooling racks to cool to room temperature.

Keeping: The cookies can be kept in a tin at room temperature for about 5 days or wrapped airtight and frozen for up to 1 month.