Preheat the oven to 350°F (175°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper.

Finely chop the 4 oz semisweet chocolate bar and melt in a microwave-safe bowl in bursts, stirring until smooth. Allow to cool slightly.

In a large bowl, cream together ½ cup unsalted butter, ½ cup oil, and 1¾ cups granulated sugar until light and fluffy, about 2–3 minutes.

Add 4 large eggs one at a time, mixing briefly until just combined. Optionally stir in vanilla extract.

In another bowl, whisk together 3 cups all-purpose flour with baking powder and salt.

Alternate adding the dry mixture and 1¼ cups buttermilk to the creamed mixture in three additions, starting and ending with dry ingredients.

Divide the batter into two bowls and stir the melted chocolate into one portion.

Spoon alternate dollops of vanilla and chocolate batter into the prepared pans. Swirl gently with a knife to form ribbons.

Bake for 25–30 minutes or until a toothpick comes out clean. Allow cakes to cool in pans for 10 minutes before transferring to a rack.

Melt 1 cup semisweet chocolate chips and let cool slightly before mixing in with 1 cup unsalted butter until smooth.

Gradually add powdered sugar until the buttercream reaches a spreadable consistency.

Level the cooled cakes if necessary. Spread a thin layer of buttercream between the layers, stack them, then apply a crumb coat and chill for 15 minutes before adding the final buttercream layer.