Preheat the oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats. Set aside.

Whisk the flour, cocoa powder, baking soda, and salt together.

In a large bowl using a hand-held or stand mixer fitted with a paddle attachment, beat the butter for 1 minute on medium-high speed until completely smooth and creamy. Add the brown sugar and beat on medium high speed until fluffy and combined. Beat in the egg and vanilla on high speed, scraping down the sides and bottom of the bowl as needed to combine.

On low speed, add half of the dry ingredients to the wet ingredients. Then add half of the buttermilk. Repeat with the remaining dry ingredients and buttermilk until everything is added. Beat on medium speed until combined. It will be a thick, tacky, and sticky cake batter.

Scoop mounds of batter, about 1 and 1/2 Tablespoons each, onto prepared baking sheets—about 3 inches apart.

Bake the cookies for 10-12 minutes or until the tops spring back when touched. Mine usually take 11 minutes. Allow to cool completely before sandwiching. Make the frosting as they cool.

Melt the butter in a small saucepan over medium heat. Once melted, add brown sugar and 2 Tablespoons of heavy cream. Whisk constantly until sugar is dissolved. Add salt. Allow to bubble for about 2-3 minutes, whisking every 30 seconds. Remove from heat, pour into a heat-proof mixing bowl, and allow to cool for about 30 minutes. With a hand or stand mixer fitted with a whisk attachment, beat in 2 cups confectioners’ sugar and 3 more Tablespoons of heavy cream. Slowly add 1/4 cup – 1/2 cup more confectioners’ sugar until you reach the desired consistency. Add 1 more Tablespoon heavy cream if you find the frosting too thick. (Or more if it’s much too thick!)

Pair the cookies up based on their size. Spread the frosting onto the flat side of one cookie and sandwich with the other. Repeat with remaining. Cover leftover whoopie pies and store in the refrigerator for up to 1 week.