YIELD: about 2 dozen filled cookies
TOTAL TIME: 2-1/2 hours, including chilling time
DIFFICULTY: 3
SPECIAL EQUIPMENT: food processor or mini food processor, stand mixer or electric hand mixer
COOKIES
1 cup (3-1/2 ounces) rolled oats
3 cups (12-3/4 ounces) unbleached all-purpose flour
1 teaspoon baking soda
1/2 teaspoon kosher salt
1/4 cup whole or reduced-fat milk
2 tablespoons (1-1/2 ounces) molasses
1 teaspoon vanilla extract
8 tablespoons (4 ounces) chilled unsalted butter, cut into 1/2-inch cubes
1 cup (7-1/2 ounces) packed light brown sugar
1/2 cup (3-1/2 ounces) granulated sugar
2 large eggs
FILLING
2 tablespoons cream cheese, softened
2 tablespoons vegetable shortening
1/2 teaspoon vanilla extract
1/2 cup (2 ounces) powdered sugar
2 tablespoons light corn syrup
2 large egg whites
1/2 cup (3-1/2 ounces) granulated sugar
1/4 teaspoon cream of tartar
MAKE THE COOKIES:
Pulse the oats in a mini food processor for about 30 seconds, until coarsely ground with some chunks remaining. Transfer to a large bowl and whisk with the flour, baking soda, and salt. Set aside.
Whisk the milk, molasses, and vanilla together in a small bowl. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar, and granulated sugar together for 2 to 3 minutes on medium speed, until the mixture is fluffy and light beige. Reduce the mixer speed to low and add the eggs one at a time, mixing thoroughly before adding the next. Add the milk and stir until fully incorporated.
Add the flour-oat mixture incrementally to form a soft, sticky dough. Cover and refrigerate for at least 1 hour.
Preheat the oven to 350°F. Line 2 baking sheets with parchment paper or Silpat liners.
Spoon the chilled cookie batter in heaping tablespoons onto the prepared baking sheets, leaving at least 1 inch space between cookies. Bake for 10 minutes, then gently smash each puffed cookie with a spatula turner to flatten. Bake for 2 to 4 more minutes; the cookies will still be soft but not gooey in the middle.
Remove the cookies from the oven and let cool on the baking sheet for 1 minute before transferring to a wire cooling rack to cool completely. Repeat with the remaining cookie dough.
FILL THE COOKIES:
Beat the cream cheese, shortening, and vanilla together in a large bowl until homogenous. Add the powdered sugar and beat to make a soft frosting. Beat in the corn syrup.
Fill a small, straight-sided saucepan halfway with water and bring to a simmer.
Place the egg whites and granulated sugar in a heatproof stainless steel or Pyrex bowl and set over the saucepan of simmering water. Whisk continuously for 1 to 2 minutes, until the sugar dissolves and the liquid is slightly opaque, frothy, and warm to the touch.
Transfer the whisked egg whites to the bowl of a stand mixer fitted with the whisk attachment. Whip on medium-high speed for 2 to 3 minutes, until the liquid becomes opaque and glossy. Add the cream of tartar and whip for 1 or 2 minutes more, until stiff peaks form when the mixer is turned off and the whisk is lifted.
Spoon the cream cheese frosting into the marshmallow mixture and whip together. Chill for at least 1 hour to firm.
Spread the chilled frosting evenly across the flat (bottom) sides of half the cookies. Top with the rest of the cookies, flat sides down, to make oatmeal creme pies.
Store at room temperature in an airtight container for up to a week.