For the Cookies • 1 cup unsalted butter melted and cooled until no longer warm to the touch (226g) • 1 cup dark brown sugar packed (200g) • ½ cup granulated sugar (100g) • 2 Tablespoons unsulphured molasses • 2 large eggs room temperature preferred • 1 ½ teaspoons vanilla extract • 2 cups all-purpose flour (250g) • 2 teaspoons cornstarch • 1 ¼ teaspoon baking soda • ¾ teaspoon ground cinnamon • ¾ teaspoon table salt • 2 cups instant/quick oats (180g)

For Marshmallow Cream Filling • 6 large egg whites (210g)  (I do not recommend using egg whites from a carton, as people have reported mixed results with these) • 1 ½ cups granulated sugar (300g) • ½ teaspoon cream of tartar • ¼ teaspoon salt • 1 ½ teaspoons vanilla extract

Combine butter, sugars, and molasses and stir well  (I do not use an electric mixer for this recipe and just stir everything by hand).

Add egg and vanilla extract and stir to combine.

In a separate bowl, whisk together flour, cornstarch, baking soda, cinnamon, and table salt until completely combined.

Gradually stir dry ingredients into butter mixture until thoroughly combined.

Stir in oats until well-distributed.

Drop batter by level 1 ½-Tablespoon-sized scoop onto prepared baking sheet and bake on 350F (175C) for 10 minutes or until edges turn golden brown.

Prepare a double boiler.  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.

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.

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 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.

Stir in vanilla extract.

Once cookies have cooled, scoop a generous portion of marshmallow cream filling onto the bottom of one cookie.  Top with another cookie to make a sandwich.