Preheat your oven to 350 degrees F (176 C) and line two baking sheets with parchment paper. Set aside.
COOKIES: To a medium bowl, add peanut butter, maple syrup, melted coconut oil, baking soda, and sea salt. Whisk well to combine. Next, add in the almond flour, oat flour, and arrowroot flour. Stir well with a wooden spoon or spatula until all of the flour is incorporated.
To make the classic Nutter Butter shape, scoop 1/2 tsp of the dough, roll it into a ball, and place it on the baking sheet. Scoop out another 1/2 tsp, roll it into a ball, and place it so it’s touching the other piece of dough. Use a fork to press into the dough horizontally then vertically to create the classic Nutter Butter hatch marks. Repeat with the remaining dough, placing the individual cookies 1 inch apart. Alternatively, scoop out 1 tsp of dough, roll it into a ball, and press it down with the prongs of the fork horizontally then vertically. They will just be little circle shapes instead of being shaped like peanuts (still equally delicious)!
Bake the cookies on the center rack for 9-11 minutes — the edges should be golden and feel set but still slightly soft. Let cool on the pan for 5 minutes before transferring to a wire rack to cool completely, about 10-15 minutes.
FILLING: To a small mixing bowl, add the peanut butter, maple syrup, and sea salt. Use a spoon or spatula to mix into a slightly thickened mixture, then set aside.
Once the cookies are cooled, place a heaping 1/2 tsp of peanut butter filling onto the flat side of one cookie and sandwich it with another (hatch-marked sides out). Repeat with the remaining cookies.
Store any leftover cookies in an airtight container at room temperature for up to 3-5 days or in the freezer for up to 1 month.