The stacks of limited edition Oreo flavors at the end of every supermarket aisle are almost impossible to resist. The ingenious seasonal offerings like this year's S'mores-flavored cookies are the perfect summer splurge. But when it's really time to impress, it's not hard at all to make a homemade version of America's favorite cookie!
The secret to getting that crispy graham cracker flavor? A healthy dose of cinnamon and a few scoops of graham flour—a type of whole wheat flour that's milled differently than regular whole wheat flour, and that is widely available via Bob's Red Mill or King Arthur Flour. If you can't find it in your local store, feel free to substitute whole wheat pastry flour.
Make the cookies:
Whisk both flours, baking powder, cinnamon, and salt together in a medium mixing bowl and set aside.
Beat the butter, shortening, and sugar together with a stand mixer or an electric hand mixer at medium speed for 3-4 minutes until the butter is whipped, fluffy, and pale.
Reduce the mixer speed to low and add the egg, stirring until fully incorporated, then the vanilla extract.
Add the reserved flour 1/4 cup at a time until fully incorporated into the dough.
Divide the dough into four equal pieces and wrap each tightly in a piece of plastic wrap.
Refrigerate the dough for at least 2 hours until fully chilled and firm, preferably overnight for best results.
Preheat the oven to 350 degrees F and line two baking sheets with parchment paper or silicone liners.
On a floured work surface, gently roll one of the dough discs until it is 1/4 inch thick. You may need to sprinkle more flour over the dough as you roll to prevent sticking.
Use a 1 1/2-inch fluted or smooth-edged cookie or biscuit cutter to cut out as many cookies as you can, re-rolling the dough scraps once if needed to get about 24 rounds. Transfer the rounds to the prepared baking sheet and repeat with the remaining dough.
Bake for about 10 minutes, just until the cookies are no longer shiny on top but barely starting to color around the edges. They will seem under-baked, but they will continue to firm up as they cool.
Transfer the cookies to wire racks and cool completely.
Make the filling:
Mix the powdered sugar, corn syrup, vegetable shortening, vanilla extract, and milk with a stand mixer or an electric hand mixer on low speed until combined, about 30 seconds.
Increase the speed to medium and beat until a thick filling forms.
Transfer half the filling to a clean bowl and add the cocoa powder to the remaining filling. Mix with the mixer or a spatula until completely incorporated.
Pinch off a piece of each flavor of filling and roll each into a ball about 1/2 inch in diameter. Place both fillings, one on top of the other, onto the underside of a cookie. Place another cookie on top of the filling balls and press gently to flatten.
Repeat with the remaining cookies and filling.