Step 1

Set oven racks in upper and lower third of the oven and preheat oven to 375 degrees. Line 18 by 13-inch baking sheets with parchment paper, set aside.

Step 2

In a medium mixing bowl whisk together flour, cocoa powder, baking soda and salt. Set aside.

Step 3

In a large mixing bowl using an electric hand mixer or in the bowl of a stand mixer fitted with the paddle attachment mix together brown sugar, buttermilk, vegetable oil, egg and vanilla extract until well blended.

Step 4

Add flour mixture and mix just until combined, then pour in hot water and mix just until combined.

Step 5

Scoop batter out using a medium cookie scoop or 1 1/2 Tbsp at a time, and drop onto prepared baking sheets, spacing 2-inches apart.

Step 6

Bake first two sheets at a once, rotating sheets halfway through baking, until cookies spring back when touched or toothpick inserted into center comes out clean, about 8 - 10 minutes total.

Step 7

Let cool about 5 minutes then use a spatula to transfer to a wire rack to cool completely. Bake remaining last sheet of cookies in center of the oven, about 8 - 10 minutes.

Step 8

In the bowl of an electric stand mixer fitted with the paddle attachment (or using a hand mixer) cream together salted butter, unsalted butter and powdered sugar on low speed until well combined.

Step 9

Increase mixer speed to high and whip until until light and fluffy, about 4 minutes. Blend in vanilla.

Step 10

Remove bowl from stand mixer, add marshmallow fluff and fold together using a rubber spatula just until combined.

Step 11

Chill mixture about 20 minutes to thicken up then fold again. Transfer to a piping bag fitted with a large round tip and pipe over half of the cookies, sandwich a second cookie over the top.

Step 12

Store cookies in an airtight container set on parchment paper, in a single layer in the refrigerator up to 3 days. Let rest at room temperature about 30 minutes before serving.