Preheat oven to 350F and generously grease and flour two 8" round cake pans (you may instead use 9" pans, just keep in mind you will need to bake the cake for about 5 minutes less than indicated). Set aside.
Place chopped chocolate in a microwave-safe bowl and microwave for 30 seconds. Stir well, and then return to microwave and heat in 15-second increments, stirring well in between, until chocolate is completely melted and smooth. Set aside.
Using an electric or stand mixer, cream together butter, oil, and sugar until light and fluffy (about 30 seconds on high speed).
Add eggs, one at a time, beating well after each addition. Pause as needed to scrape down sides and bottom of bowl.
Stir in vanilla extract.
In a separate, medium-sized bowl, whisk together flour, baking powder, and salt.
Using a spatula and gently hand-mixing, alternate adding flour mixture and buttermilk to the butter/oil batter, starting and ending with flour mixture and mixing until just combined after each addition. Do not overmix!
Pour 1/3-1/2 of the batter into a separate bowl and add chocolate mixture. Stir until completely combined and batter is evenly chocolate.
Alternate adding vanilla and chocolate batter to each pan, evenly distributing batter into each pan. Use a knife to swirl for marbled appearance.
Bake on 350F (175C) for 30 minutes, or until toothpick inserted in center comes out mostly clean but with a few moist crumbs (do not over-bake or the cake will be dry, you want there to be a few crumbs).
Allow cake to cool for 15 minutes before carefully inverting onto cooling rack to cool completely before 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.
¹Please note that in the video I frost the cake with my chocolate buttercream frosting, which is a bit quicker and simpler than the chocolate frosting I have used here. You can't go wrong with either one.
Cake flour may be substituted. If using weights, use the same weight indicated in the recipe. If using cups you will need to use 3 cups + 6 Tablespoons of cake flour (cake flour is lighter than all purpose, which is why the weight is the same but the volume is different).
Store cake in an airtight container at room temperature for up to 2 days or in the refrigerator for up to 5 days. Keep in mind the refrigerator tends to dry out cakes and make them a bit dry!
A cake that turns out dense, dry, or crumbly (or tasting like cornbread) was most likely over-mixed, the flour was over-measured, or the cake was over-baked. When mixing wet and dry ingredients together, do not use an electric mixer, use a spatula and a gentle hand to stir ingredients together until combined.If you are not using a scale for this recipe, please see my tutorial on how to measure flour.When baking, always bake in the center rack. Make sure that your oven is not running hot, as many ovens run much hotter or cooler than indicated. Keep an oven thermometer near the center of your oven to monitor the temperature, as an oven that runs hot will cook a cake too fast and dry it out. Use the toothpick test to test if your cake is done. Ideally a toothpick inserted in the center of the cake should come out with a few moist crumbs (but not wet batter). If the toothpick comes out completely clean, the cake is definitely done baking and may be bordering on being over-baked.