Preheat and prep: Set your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. If using 9-inch pans, you'll need to reduce the baking time by about 5 minutes.

Melt the chocolate: Place your chopped semisweet chocolate in a microwave-safe bowl. Heat in 30-second intervals, stirring between each until completely melted and smooth. Set aside to cool slightly.

Cream the butter mixture: In a large bowl, beat together the softened butter, oil, and sugar until light and fluffy, about 30 seconds on high speed.

Add eggs and vanilla: Add the eggs one at a time, beating well after each addition. Don't forget to scrape down the sides of the bowl as needed. Stir in the vanilla extract.

Prepare dry ingredients: In a separate bowl, whisk together the flour, baking powder, and salt.

Combine wet and dry ingredients: Alternately add the flour mixture and buttermilk to the butter mixture, beginning and ending with the flour. Mix until just combined – this is crucial! Switch to a spatula and fold by hand to avoid overmixing, which can lead to a dense cake.

Create the marble effect: Pour about ⅓ to ½ of the batter into a separate bowl. Add the melted chocolate to this portion and stir until completely uniform.

Fill the pans: Alternately add spoonfuls of vanilla and chocolate batter to your prepared pans. Once all the batter is in the pans, use a knife to gently swirl the batters together. Don't overdo it – a few swirls will create the most distinct marble pattern.