Preheat the oven to 350F and line a large baking sheet with parchment paper.
Add the softened butter and granulated sugar to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy-- about 2 minutes.
In another bowl, whisk together the flour, cocoa powder, and baking soda.
In a third bowl, whisk together the buttermilk, egg, and vanilla.
With the mixer on a low speed, alternate adding the dry ingredients and wet ingredients in batches. I add ⅓ of the dry, ½ of the wet, ⅓ of the dry, last ½ of the wet, and last ⅓ of the dry-- but it doesn’t have to be exact. You just want it to all get mixed in evenly.
Scrape down the sides of the bowl to ensure that all of the ingredients are well mixed.
Scoop the batter into small portions onto the parchment lined baking sheet making sure to place them at least two inches apart. I use a 2 tbsp cookie scoop to ensure they’re all the same size!
Bake for 12-14 minutes or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool on the baking sheet and while you make the filling.