Coconut-Chocolate Pinwheel Cookies

In a medium bowl, whisk together flour, baking soda, and salt. In the bowl of a mixer fitted with the paddle attachment, beat together 8 tablespoons butter and sugar on medium speed until light and fluffy, about 3 minutes. Beat in egg and vanilla. Reduce speed to low and slowly add flour mixture, beating until almost combined. Add coconut and beat until incorporated. Place dough between two sheets of parchment and roll out to a 10 1/2-by-12 1/2-inch rectangle, 1/4 inch thick. Using a sharp knife, trim 1/2 inch from edges to create a 10-by-12-inch rectangle. Transfer to a baking sheet. Refrigerate until firm, at least 1 hour and up to overnight.

Place chocolate and remaining 1 tablespoon butter in a heatproof bowl set over a pot of simmering water; stir until melted and smooth, about 3 minutes. Remove from heat and stir in condensed milk. Let cool 5 minutes. Mixture will be thick and pasty.

Remove dough from refrigerator and peel off top sheet of parchment. Using an offset spatula, spread chocolate mixture evenly over surface of dough. Using the bottom sheet of parchment for support, roll dough into a tight log. Wrap log in plastic; refrigerate at least 2 hours and up to overnight.

Preheat oven to 350 degrees. Remove plastic from log and cut log into 1/4-inch-thick rounds. Transfer to parchment-lined baking sheets, spaced 1 inch apart. Bake, rotating pans halfway through, until firm and beginning to brown around edges, 15 to 18 minutes. Let cool on sheets 5 minutes, then transfer cookies to wire racks and let cool completely. Cookies can be stored in an airtight container at room temperature up to 1 week.