Start by preheating your oven to 375 degrees and spray your cookie sheet with non-stick spray or cover with a sheet of parchment paper.
In a large mixing bowl, combine melted butter with the pineapple cake mix, all purpose flour, pineapple juice or pineapple extract and eggs. When you have your ingredients combined, refrigerate dough for at least 30 minutes.
Once chilled, remove from refrigerator and spooning up enough dough to roll into a 1 inch ball. Move your cookie to your prepared cookie sheet and repeat until all the dough has been used. When your sheet is full, cook them for 9-11 minutes. They will be puffy and crinkly and beautiful.
Remove them from the oven, let them cool for one minute and then move them to a cooling rack.
Once your pineapple cookies have cooled completely, start your coconut buttercream frosting. Add softened butter to a large mixing bowl and using an electric mixer, beat butter until smooth and creamy. To the butter add coconut extract, salt and milk, then slowly mix in the sifted powdered sugar in batches until all has been incorporated.
Spoon frosting onto the back of one pineapple cookie and then top it off back to back with another pineapple cookie, making a sandwich.