Position 2 racks in the middle of the oven and preheat to 400 degrees F. Line 2 baking sheets with parchment paper or silicone baking mats (If you only have 1 baking sheet, let it cool completely between batches).
Whisk the flour, baking powder, baking soda and salt together in a medium bowl.
Melt the butter in medium saucepan over low heat, remove the pan from the heat, and stir in the brown sugar with a wooden spoon (the sugar will get soft and saucy, but there may still be a layer of butter on top; don't worry, the mixture comes together as you add the rest of the ingredients).
Stir in the egg, then the flour mixture, taking care not to overmix.
Stir in the coconut, oats and nuts to make a textured cookie dough.
Set aside until it gets to room temperature, about 30 minutes.
Using a tablespoon-sized cookie scoop, or a tablespoon,scoop balls of the dough onto the prepared pans, arranging them about 2 1/2 inches apart. Bake until the cookies are golden brown with toasted edges, about 12 minutes.
Cool cookies in the pans for a couple minutes then transfer with a spatula to racks to cool.
You can store unshaped dough for up to 3 days in the refrigerator. Store the cookies in airtight containers for up to a week. Freeze for 1 month for longer storage.