1-Preheat oven to 350°F (180°C), or 320°F (160°C) with fan on. Grease and butter or spray a 10-inch bundt pan or two 8-inch round cake pans thoroughly. Reduce temperature to 325°F (165°C) if using dark pans.

2-Melt chopped chocolate with coffee and oil or butter in a heatproof bowl over simmering water or microwave in short bursts. Cool to room temperature.

3-In a separate bowl, sift or whisk together flour(s), baking powder, baking soda, and salt.

4-Beat butter and oil (if using both) with an electric mixer until smooth and creamy. Gradually add sugar and beat until light and fluffy (3–5 minutes).

5-Add eggs one at a time, beating well after each. Incorporate vanilla extract, white vinegar (if using), and sour cream or yogurt.

6-On low speed, alternately add flour mixture and buttermilk in three parts, starting and ending with flour. Fold gently as needed to avoid toughening.

7-Remove a little more than half the batter into a separate bowl. Stir melted chocolate or cocoa powder into this portion until uniform.

8-Alternately spoon vanilla and chocolate batters into the prepared pan(s). Gently swirl using a skewer or knife in a figure-8 or marble pattern without overmixing.

9-Bake 30 to 40 minutes, depending on pan size and oven, until a toothpick inserted in center comes out clean or with a few moist crumbs. Avoid overbaking for moist cake.

10-Cool in pans 15–20 minutes, then transfer to a wire rack to cool completely before serving or frosting.