Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a bowl, whisk together flour, baking soda, and salt. Set aside.
In another bowl, beat the butter and both sugars until light and creamy. Add the egg, vanilla, and rum, mixing until smooth.
Gradually add the dry ingredients and stir just until combined. Fold in the shredded coconut and any optional mix-ins.
Scoop tablespoon-sized portions onto the baking sheet, leaving space between each cookie.
Bake for 10 to 12 minutes, or until the edges are golden and the centers are soft.
Let the cookies cool on the tray for a few minutes before transferring them to a rack.