Make the cookie dough: In a large bowl, combine the flour, salt, cinnamon, oats and baking powder. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, or in a separate large bowl with an electric hand mixer, cream together the butter and sugar for about 2 minutes on medium speed, until well-combined.
Add the eggs, 1 at a time, incorporating each egg before adding the next. Scrape down the sides of the bowl.
Slowly add the dry ingredients to the butter mixture and beat until just combined. Scrape down the sides of the bowl if needed.
In a small bowl, combine the boiling water and baking soda. Add to the dough and beat thoroughly to combine.
Chill the dough in the refrigerator for 4 hours.
Preheat the oven to 350˚F (180˚C).
Make the maple buttercream: In the bowl of a stand mixer fitted with a paddle attachment, or in a large bowl with an electric hand mixer, cream the butter on medium speed until smooth and pale in color. Scrape the sides of the bowl down with a spatula as needed.
With the mixer running, slowly drizzle the maple syrup into the butter. Beat until well-combined.
Add the salt and vanilla. Mix well, scraping down the sides of the bowl as needed.
Add the powdered sugar and beat until well-combined. Set the buttercream aside.
Scoop ¾-ounce (20 grams) balls of the chilled dough and onto a parchment-lined baking sheet, spacing about 3 inches apart. (You may need to bake in multiple batches). Moisten your fingers with water and gently flatten each scoop of dough into an even round about ¼-inch (½ cm) thick.
Bake the cookies for 8-10 minutes, until the edges are golden brown and the centers are dry, but still chewy and pliable. Using a small spatula, gently coax any overly spread cookies back into a round shape.
Let the cookies cool completely.
Fill a piping bag with the maple buttercream. Pipe a flat round of buttercream onto half of the cookies. Then, top with the other cookies.
Freeze any extra cookies in an airtight container for up to 10 days. Exposure to moisture in the air both in the freezer and at room temperature will cause the cookies to get sticky and soggy.
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