Preheat oven to 350°F and line a large baking sheet with parchment paper. Set aside
Add the 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.
10 tablespoons unsalted butter, 1¼ cups granulated sugar
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.
2¼ cups all-purpose flour, ½ cup unsweetened cocoa powder, 1 teaspoon baking soda, ½ teaspoon kosher salt
In a small bowl, whisk together the buttermilk, egg, and vanilla.
1 cup buttermilk, 1 large egg, 1 teaspoon pure vanilla extract
With the mixer on low speed, alternate adding the dry ingredients and wet ingredients in batches to evenly incorporate: add ⅓ of the dry, ½ of the wet, ⅓ of the dry, last ½ of the wet, and last ⅓ of the dry. Scrape down the sides of the bowl to ensure that all of the ingredients are well mixed.
Use a 2-tablespoon-sized cookie scoop to scoop the batter into small mounds, 2 inches apart, on the prepared baking sheet.
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.