1 cup (2 sticks) unsalted butter, softened3/4 cup granulated sugar 1 large egg 1 egg white, beaten 1 teaspoon pure vanilla extract 2 1/2 cups sifted all-purpose flour 1 tablespoon finely chopped fresh rosemary 3/4 teaspoon coarse salt 1/2 cup fine sanding sugar
Put butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment; mix on medium speed until pale and fluffy, about 2 minutes. Mix in whole egg and vanilla. Reduce speed to low. Add flour, rosemary, and salt, and mix until combined.
Halve dough; shape each half into a log. Place each log on a 12-by-16-inch sheet of parchment. Roll in parchment to 1 1/2 inches in diameter, pressing a ruler along edge of parchment at each turn to narrow log. Transfer to paper-towel tubes to hold shape, and freeze until firm, about 1 hour.
Preheat oven to 375. Brush each log with egg white; roll in sanding sugar. Cut into 1/4-inch-thick rounds. Space 1 inch apart on baking sheets lined with parchment. Bake until edges are golden, 18 to 20 minutes. Let cool on sheets on wire racks. Store in airtight containers at room temperature up to 3 days.