Preheat oven to 350 degrees. Line a large baking sheet with parchment paper or a silicone baking mat.
Use electric mixer to beat brown sugar, sugar, and butter until lightened and fluffy, about 3 minutes.
Mix in eggs, one at a time, scraping down sides as needed. Then, mix in vegetable oil and vanilla extract.
In a small bowl, whisk together flour, cocoa powder, baking soda and salt.
Alternate mixing in flour mixture and buttermilk in 3-4 additions each, beginning and ending with flour mixture.
On prepared baking sheet, drop about 3 tablespoons of batter in a circle shape. Make sure that edges of circles are at least 2 inches apart.
Bake for 10-12 minutes, or until tops of cakes have a cracked appearance and a toothpick inserted into the center one of the center cakes comes out with just a few crumbs.
Let cook on pan for 2-3 minutes before transferring to a cooling rack. Let cool completely before filling.