In a medium bowl, whisk together the flour, salt, baking powder, baking soda and cinnamon; set aside.
In the bowl of a stand mixer or with a handheld electric mixer, cream the butter and sugars until light an fluffy; about 4 minutes. Scrape the sides of the bowl as needed.
Add the eggs, egg yolk and extracts and mix until well blended and creamy; about 2 minutes. Scrape the sides of the bowl as needed.
On low speed, add in the flour mixture and blend until just barely combined.
Continue to mix on low speed while adding the oats and coconut. Mix until blended.
Scoop dough out into 3 Tbsp. sized doughballs and place them on a parchment lined baking sheet. Leave 3 inches in between each cookie. Slightly flatten each cookie with the palm of your hand.
Bake for 10-12 minutes or until the edges begin to brown.
Remove from oven and allow to cool.
Prepare filling:
With a mixer, cream together the butter and shortening.
Add the extracts and milk and blend in on low speed for 30 seconds. Scrape the sides of the bowl and mix again on medium speed until blended and creamy.
Slowly add in the powdered sugar until well blended and fluffy.
Spread or pipe the filling onto the bottoms of half the cookies.
Top each cookie with an unfrosted one and press slightly to make a sandwich.
Keep stored in an airtight container, or individually wrap each cookie sandwich.