For CookiesPreheat oven to 350F (175C) and line a baking sheet with parchment paper. Set aside.

Combine butter, sugars, and molasses and stir well (I do not use an electric mixer for this recipe and just stir everything by hand).1 cup (226 g) unsalted butter, 1 cup (200 g) dark brown sugar, ½ cup (100 g) granulated sugar, 2 Tablespoons unsulphured molasses

Add egg and vanilla extract and stir to combine.2 large eggs, 1 ½ teaspoons vanilla extract

In a separate bowl, whisk together flour, cornstarch, baking soda, cinnamon, and table salt until completely combined.2 cups (250 g) all-purpose flour, 2 teaspoons cornstarch, 1 ¼ teaspoon baking soda, ¾ teaspoon ground cinnamon, ¾ teaspoon table salt

Gradually stir dry ingredients into butter mixture until thoroughly combined.

Stir in oats until well-distributed.2 cups (180 g) instant/quick oats

Drop batter by level 1 ½-Tablespoon-sized scoop onto prepared baking sheet and bake on 350F (175C) for 10 minutes or until edges are beginning to turn golden brown. Allow cookies to cool completely on baking sheet before filling.

For Marshmallow Cream Filling (see note for alternative filling options)Before you begin: You will need a double boiler for this recipe. I recommend using the metal bowl of a stand mixer and a saucepan that is large enough to support the bowl without it touching the bottom of the pan. While this recipe can be prepped with an electric hand mixer, please note that it will be quite an arm workout and will take a long time and I recommend using a stand mixer if you have one).

In the completely clean, dry and grease-free bowl of a stand mixer, combine egg whites, sugar, cream of tartar and salt and whisk to combine.6 (210 g) large egg whites, 1 ½ cups (300 g) granulated sugar, ½ teaspoon cream of tartar, ¼ teaspoon salt

Bring 1 ½-2” (4-5cm) of water to a simmer over medium-low in a medium-sized saucepan (use a saucepan that your mixing bowl will fit in without touching the bottom of the pan).

Place mixing bowl over simmering water and whisk constantly until the sugar is dissolved (if you rub a small bit of the syrup between your fingers (be careful, it’s hot!) it should no longer feel gritty at all) and the mixture reaches at least 160F (70C).

Remove mixing bowl from heat and fit it into your mixer. Using a whisk attachment, beat the egg whites on medium-high speed until mixture becomes thick and glossy and you have billowy stiff peaks. This typically takes approximately 5-10 minutes with a stand mixer but will take longer if you are using a hand mixer with beaters.

Stir in vanilla extract.1 ½ teaspoons vanilla extract

AssemblyOnce cookies have cooled, scoop a generous portion (I use about 2 Tablespoons) of marshmallow cream filling onto the bottom of one cookie. Top with another cookie to make a sandwich/marshmallow cream pie. Serve and enjoy!