Combine all ingredients in a medium bowl and whisk. Refrigerate until ready to use.

In a bowl, sift together the flour, salt, baking soda and baking powder. Stir in oats and set aside.

In the bowl of a stand mixer with the paddle attachment, on medium speed, cream together butter, peanut butter, coconut oil and sugars until pale, for 3-4 minutes. Scrape down the mixing bowl as needed.

Turn the mixer down to low and add the eggs, one at a time. Add vanilla and mix to combine.

Add dry ingredients in 3 additions and mix on low speed. Mix until just combined. Do not overmix. Scrape down the bowl, to ensure everything is mixed well.

Use a small ice cream scoop (if you have one) or 1 heaping tablespoon of dough per cookie. Place scoops on a parchment lined cookie sheet. You'll have between 40 and 48 cookies. Chill for at least 2 hours. (For chilling you can place the cookies as close together as possible, but for baking they need to be 1 inch apart).

To bake the cookies: Preheat oven to 375 F. Place 8 cookies on each parchment-lined sheet. There is no need to flatten cookies, they will spread while they bake. Set the baking sheet inside another one to double-pan. This will ensure even baking and spreading. Bake until evenly golden for about 12-14 minutes, rotating the pan half-way through. Remove pan from the oven and cool for 10 minutes. Then remove cookies from the pan and let them cool completely on a cooling rack.

Turn one cookie flat side up and add 2 tsp of filling, then top with another cookie, flat side down. Repeat until all cookies are assembled.