Beat 8 tablespoons of the butter and the sugar on medium speed until fluffy. Add the egg and vanilla, and beat until combined.

In a medium bowl, whisk flour, baking soda, and salt. Add flour mixture to butter mixture, and beat on low speed until combined.

Add coconut, and beat until combined.

Roll out the dough between two 12" x 17" pieces of parchment paper into a 10 1/2" x 15 1/2" rectangle, a scant 1/4" thick.

Using a sharp knife, trim to even dough to a 10" x 15" rectangle. Transfer dough to a baking sheet, and place in refrigerator to chill for at least 1 hour.

Melt the chocolate and the remaining tablespoon of butter on the stove and remove from heat.

Stir in the condensed milk. Let stand until slightly thickened, about 5 minutes. (For best results, use immediately.)

Remove dough from the refrigerator. Peel off the top piece of parchment.

Using an offset spatula, spread the melted chocolate mixture over the dough. Using the bottom piece of parchment for support, roll the dough into a log.

Wrap the log in parchment, and chill overnight.

Preheat the oven to 350 degrees.

Remove the parchment from the log, and cut log into 1/4" thick rounds. Place on cookie sheets and bake until light golden brown on edges, 8-10 minutes.

Transfer cookies to a wire rack to cool. Bake or freeze remaining dough.

Store the cookies in an airtight container for up to 2 weeks.