Preheat the oven to 350°F (175°C) to ensure even baking.
In a large mixing bowl, use an electric mixer to cream together the butter, brown sugar, and white sugar until light and fluffy, about 2-3 minutes.
Beat in the egg and vanilla extract until well combined, adding moisture and flavor to the dough.
In a separate bowl, whisk together the all-purpose flour, kosher salt, and baking soda. Gradually add this to the wet ingredients, mixing until just combined.
Gently fold in the coconut M&Ms using a spatula, being careful not to overmix to keep the M&Ms intact.
Using a tablespoon or cookie scoop, drop dough mounds onto a parchment-lined baking sheet, spacing about 2 inches apart.
Bake for 10-12 minutes until edges are lightly golden; centers may look slightly underbaked but will firm up as they cool.
Remove from oven and let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.