In a medium bowl, use an electric mixer to beat together the cold salted butter, granulated sugar, light brown sugar, and vanilla extract. Mix on medium speed until light, creamy, and fluffy, about 3-5 minutes.

Gradually sift in the all-purpose flour, unsweetened shredded coconut, and ground cinnamon into the creamed mixture. Switch to low speed, blending until just combined.

Transfer the dough onto a clean surface, divide into two halves, and roll into logs approximately 1½ – 2 inches in diameter. Wrap each log in plastic wrap and chill in the refrigerator for at least 1½ hours.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

Remove the dough logs from the refrigerator, unwrap, brush with a well-beaten egg, and roll in extra shredded coconut until fully coated.

Slice each log into ¼-inch thick rounds, dipping the tops in sanding sugar. Arrange on baking sheets and bake for 10-12 minutes, or until edges turn lightly golden.