3/4 cups macadamia nuts, toasted and roughly chopped
2 cups marshmallow fluff
Place racks in the center and upper third of the oven and preheat oven to 350°F. Line two sheets with parchment paper and set aside.
Beat the sugar and butter in the bowl of a stand mixer fitted with a paddle attachment. Beat until creamy, about 3 to 4 minutes. Add eggs and beat on medium speed for about 1 minute. Add the vanilla extract and beat until blended.
In a medium bowl, whisk together the oats, flour, baking powder, baking soda and salt. Add the dry ingredients to the butter and egg mixture slowly, beating on low speed until just incorporated. Lastly, stir in the coconut and macadamia nuts.
Measure 2 1/2 cups batter in the center of each prepared baking sheet and press down slightly to form two round, giant cookies, making sure they are fairly similar in size. (Dampen hands slightly to avoid dough sticking to them.)
Bake for 20 minutes, or until they're lightly golden brown. Allow to cool completely before filling.
Once completely cooled, place one cookie on a serving dish and spread marshmallow fluff evenly over the bottom side. Carefully top with the other cookie to form a giant cookie sandwich. Serve like a cake and slice into wedges.