In a medium-sized bowl, toss flour, oats, baking soda, and cornstarch together. Set aside.
In a large bowl or the bowl of your stand mixer fitted with the paddle attachment, cream butter and both sugars until light and fluffy, about 2 minutes. Add the eggs, egg yolk, vanilla extract, coconut extract and mix until smooth. Pour in the flour/oat mixture and mix until just combined. Do not over-mix. Stir in the toasted coconut by hand with a rubber spatula. Chill dough for 2 hours minimum.
Preheat oven to 350 degrees. Line two baking sheets with parchment paper and set aside.
Roll 2 tablespoons of dough in a ball and place on prepared cookie sheet 2" apart. Repeat this step for the remaining dough. Place the dough that is not being used back into the refrigerator until ready to use so it stays nice and cold. Bake cookies for 10-12 minutes. They will look a little wet in the center and undone but this is okay. They will firm up as they cool to room temperature - this is what keeps them soft. If you like a crunchier cookie, simply bake longer.