Whisk together the flour, baking soda, baking powder, and salt in a medium bowl; set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar and 1/3 cup of granulated sugar for 2 to 3 minutes, until pale and fluffy. Beat in the egg, followed by the maple syrup and vanilla. Slowly add the dry ingredients, mixing until just combined. Cover and chill the dough for at least 1 hour.

Preheat oven to 350ºF and line a baking sheet with parchment paper. Place the 2 tablespoons of sugar in a small bowl. Using a small cookie scoop or two spoons, scoop 1-inch balls of dough and roll each ball in the sugar.

Place dough balls on baking sheet and bake for 10-12 minutes, or until the cookies are slightly browned around the edges and puffed. Cookies will deflate some as they cool.

Remove from the oven and allow to cool on the baking sheets for 2 minutes before transferring them to a wire rack to cool completely.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and salt together on high until fluffy and pale in color, about 5-6 minutes. Decrease speed to low and slowly add the powdered sugar. Add the pumpkin purée, vanilla, and heavy cream and beat until combined. Increase speed to medium-high and beat until light and fluffy, about 4 minutes.

Spoon the buttercream into a piping bag fitted with a large round tip (or a large zip-loc bag with the tip cut off), and pipe the buttercream onto the bottom side of a cookie. Gently press another cookie on top, forming a sandwich. Repeat with the remaining cookies.