To make the graham cracker cookies:

• First, brown the butter:
  • Place 226 gm (1 cup) unsalted butter in a small stainless steel pan over medium heat, stirring until melted.
  • Allow to come to a boil, then reduce heat to medium-low, stirring frequently. The butter will start to make a crackling noise.
  • Eventually the crackling will subside, and a layer of foam will surface to the top. As you continue to stir, you’ll notice the liquid butter underneath the foam will go from golden yellow, to tan, to brown, and will have brown flecks throughout. Once you’ve reached this stage, pour the browned butter into a medium-sized heatproof bowl, being sure to get all the brown bits from the bottom of the pan. Place in the freezer for 15 minutes, until the browned butter is cooled and slightly thickened.
• While the browned butter is chilling, combine the graham cracker crumbs, flour, baking soda, kosher salt, cornstarch and ground cinnamon in a large bowl. Set aside.
• Pour the chilled browned butter into the bowl of a stand mixer fitted with the paddle attachment, being sure to scrape out all browned bits and any butter that is sticking to the sides of the bowl. Add 56 gm (1/4 cup) softened butter, brown sugar and sugar to the mixing bowl, and beat on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl. Beat in the eggs, one at a time, being sure that the first is fully incorporated before adding the next. Mix in the vanilla.
• Add the graham cracker mixture to the butter mixture and beat on low speed until just combined.
• Cover the dough and chill in the refrigerator for 1-2 hours. Meanwhile, preheat the oven to 350°F and line 2 baking sheets with parchment paper.
• Using a #40 spring-loaded cookie scoop, scoop the chilled dough and roll between the palms of your hands to form a smooth ball. Place on the prepared baking sheets, spacing 3″ apart.
• Bake for 10-13 minutes, until the edges of the cookies are set and just lightly browned. While the cookies are fresh from the oven, gently press a chocolate square onto the top of each. Allow the cookies to set on the baking sheet for 5 minutes before removing to a wire rack to cool completely. Allow the chocolate squares to set before filling with marshmallow crème.

To make the marshmallow crème filling:

• Place the egg whites, sugar, and cream of tartar in a large heatproof bowl and whisk to combine. Set the bowl over a saucepan containing 2 inches of simmering water, being careful not to let the water touch the bowl containing your ingredients.
• Whisk constantly, until the sugar dissolves and an instant read thermometer reads 160°F (71°C). This will take about 5-7 minutes.
• Remove the bowl from heat and whisk in the vanilla extract. Pour the mixture into the bowl of a stand mixer fitted with the whisk attachment. Whisk on high speed until stiff, glossy peaks form, about 5 minutes. Use immediately.

Assembly

• Transfer the marshmallow crème to a piping bag fitted with a large circular tip (I use a #806 Ateco tip for this).  Pipe the marshmallow crème onto the underside of one cookie and place a second cookie on top to form a sandwich . Repeat with the remaining cookies and marshmallow crème!