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

In a stand mixer fitted with the paddle, beat the cold butter pieces with granulated and brown sugar and the vanilla for 2–3 minutes until lighter and fluffy.

Gradually add the flour and ½ cup shredded coconut, mixing on low until the dough just comes together without overmixing.

Turn the dough onto a work surface, divide it into two equal portions, and sprinkle ⅜ tsp cinnamon over each half, pressing lightly to combine.

Shape each half into a log about 1½–2 inches in diameter, wrap tightly in plastic wrap, and chill for at least 1½ hours.

Once chilled, brush each log with the beaten egg, roll in extra shredded coconut to coat, and slice into ¼-inch rounds.

Dip the cut edges or top of each slice in sanding sugar for added sparkle.

Place cookies 1 inch apart on prepared sheets and bake for 10–12 minutes until the edges are just golden, watch closely.

Let cookies rest on the baking sheet for 5 minutes to set before transferring to a wire rack to cool completely.