In a small bowl mix milk and vinegar to create "buttermilk". Set aside.
In a stand mixer beat softened butter and brown sugar together on medium speed until fluffy. About 4 minutes.
In a separate bowl, whisk together flour, cocoa, baking soda, baking powder, and salt. Set aside.
In the mixer, beat in eggs and vanilla extract until incorporated, scraping down the bowl as needed.
Reduce mixer to low and beat in half of the flour mixture, then half of the buttermilk. Repeat with the remaining flour and buttermilk mixture. Scrape the sides as needed. Don't overmix.
Preheat oven to 375F and line baking sheets with parchment paper.
Let the batter rest for 5-10 minutes while the oven preheats.
Give it a gentle stir after the rest so there aren't big air pockets. Using a small scoop, drop batter onto prepared baking sheets. (I use a #40 scoop which is approximately 2 tablespoons of batter.)Leave 2 inches between each mound of batter so they don't spread into each other. Bake for 8-10 minutes, or until tops begin to firm up. Cool on the baking sheet for 1 minute and then transfer to cooling rack to cool completely. (In my oven they are done in 8 minutes, but I know this can vary with ovens so just watch yours to get a feel for timing. You don't want to under bake them because then they aren't sturdy and you don't want to over bake them and dry them out.) While the cakes are cooling make your filling. In a stand mixer using the whisk attachment, beat the softened butter, powdered sugar, and vanilla together until fluffy. About 2 minutes. Scrape down the sides as needed. Then add in marshmallow fluff and mix until incorporated.
Using a piping bag or spoon spread or pipe about 2 tablespoons of filling onto one cake layer and then gently press the other cake layer to make a sandwich.
