Preheat oven to 350F (175C) and prepare two deep 8" round cake pans⁴ by lining the bottoms with parchment paper and lightly greasing and flouring the sides. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, whisk together flour, sugars, cocoa powder, baking soda, and salt.
Add melted butter and oil, stir well.
Add eggs and egg yolk, one at a time, stirring well after each addition. Pause occasionally to scrape down sides and bottom of bowl.
Stir in vanilla extract. Gradually add buttermilk and stir well.
Add hot coffee, stirring until ingredients are well-combined (be sure to scrape sides and bottom of bowl again).
Evenly divide batter between prepared pans. Bake on 350F (175C) for 35-40 minutes (toothpick inserted in center should come out with moist crumbs).
Allow to cool for 15 minutes before inverting onto cooling rack to cool completely before frosting.
Chocolate Buttercream Frosting
In a microwave-safe bowl, microwave chocolate chips at 15 second intervals, stirring between intervals, until chips are completely melted. Set aside and allow to cool slightly (about 10 minutes, stirring occasionally).
Cream room temperature butter with an electric mixer. Stir in slightly cooled chocolate and beat well.
Gradually add sugar, scraping down the sides and bottom of bowl occasionally.
Sprinkle in salt and vanilla extract, stir well.
Gradually add heavy cream, increase speed to high and beat for 1 minute.
Spread a thick layer of frosting over the top of one cake round and sprinkle with mini chocolate chips. Top with second cake round and evenly frost cake. Slice and serve.
¹If you don't have buttermilk on hand, please see my post on making an easy buttermilk substitute!²You can substitute with hot water if you don't have coffee, but do make sure it is hot! See the post for more information on why I use coffee and why it is important that the liquid be hot.³I used my favorite chocolate frosting for this recipe, but here are some other frosting options you might like to try out:
⁴You can use 9" pans instead, but the cakes will need to bake for less time. I'd start checking the cakes at 27/30 minutes. Please keep in mind that if your pans are dark-colored the cake may also need less time to bake.