Preheat the oven to 325 degrees. Line two baking sheets with parchment paper.
Spread 2 cups of the coconut on a sheet pan and toast in the oven for 7-10 minutes, stirring once, or until the coconut has golden brown edges. Remove from the oven and allow to cool. Increase the oven temperature to 350 degrees.
In a mixing bowl, whisk together the flour, baking soda, cornstarch, and salt. Set aside.
In the bowl of an electric mixer, cream together the butter and sugars until smooth and creamy. Add the eggs, one at a time, beating well after each addition. Beat in the vanilla extract. Beat in the flour mixture until just combined. Stir in the toasted coconut and other additions, if desired.
Place the remaining untoasted coconut in a small bowl. Scoop out rounded tablespoons of dough, rolling the tops in the coconut before dropping them on the sheet pan. Make sure the cookies are at least 2 inches apart.
Bake for 9-12 minutes or until the edges are just starting to brown, but the centers are still very puffy and light in color. Allow to cool for a few minutes on the pan (cover with a dish towel for maximum softness), then transfer to an airtight container or ziplock bag while still warm.
