For the cookies:
8 oz. (1½ cups plus 2 tbsp.) all-purpose flour
4 oz. (1¼ cups) old-fashioned oats
½ ounce (2 tbsp.) unsweetened cocoa powder
2 oz. (1 cup, packed) dried apple rings
3 oz. (6 tbsp.) unsalted butter, at room temperature
2¼ oz. (¼ cup plus 1½ tsp.) canola oil
7 oz. (1¾ cup plus 2 tbsp.) sugar
1½ oz. (3 tbsp.) molasses
3 oz. (6 tbsp.) corn syrup
1½ tsp. baking soda
½ tsp. baking powder
½ tsp. kosher salt
¼ tsp. ground ginger
¼ tsp. ground cinnamon
¼ tsp. ground mace
1 large egg
2 ounces (¼ cup) milk
For the filling:
2 cups marshmallow creme
4 oz. (8 tbsp.) unsalted butter, at room temperature
¾ cup confectioners’ sugar, sifted
Pinch of salt
½ tsp. cream or milk
To make the cookies, preheat the oven to 350˚ F. Line baking sheets with parchment paper. In the bowl of a food processor, combine the flour, oats, cocoa powder and apples. Pulse at first to chop coarsely, then process about 2-4 minutes, until no large apple bits or full oats remain. Set aside.
In the bowl of an electric mixer, combine the butter, oil, sugar, molasses, corn syrup, baking powder, baking soda, salt, and spices. Beat on medium-high speed until well combined, about 2 minutes. Mix in the egg and beat 1 minute more. Blend in the dry ingredients and the milk just until incorporated. Scrape down the sides of the bowl and beat 30 seconds more.
Use a dough scoop to portion out the dough onto the prepared baking sheets (about 2 teaspoons for smaller cookies, about 1 tablespoon for larger cookies), leaving ample room as the cookies will spread while baking. Transfer the pans to the oven and bake, rotating halfway through, until the cookies are set around the edges and puffed up in the center, about 8 minutes for smaller cookies and 10 minutes for larger cookies. (They may seem too soft but remember that the original is a very soft cookie so that is the desired texture here. Also remember the cookies will continue to bake a bit longer once they are out of the oven.) When cool and set enough to move, transfer to a wire rack to cool completely.
To make the filling, combine the marshmallow creme and butter in a mixing bowl and beat on medium-high speed until smooth. Blend in the confectioners’ sugar and salt and whip until the mixture is light and fluffy, about 1-2 minutes more. Transfer the filling mixture to a pastry bag fitted with a medium round tip. (The tip I use is not labeled, but the opening is just under ½-inch in diameter.)
To assemble, pair the cookies up by size. Pipe a small dollop of filling onto the flat side of one cookie of each pair. Gently sandwich together with the remaining cookie, pressing the filling to the edges. Store in an airtight container.