Preheat oven to 350 °F (175 °C). Grease and flour one 9‑inch round or 8×4‑inch loaf pan.

Cream the butter and sugar together until light and fluffy, about 3–4 minutes. Add eggs one at a time, beating well after each, then stir in vanilla extract.

In a separate bowl, whisk together flour, baking powder, and salt.

Alternate adding dry ingredients and milk (plus sour cream if using) into the wet mixture, beginning and ending with the dry. Mix just until combined.

Prepare the chocolate swirl by dissolving cocoa powder in hot coffee (or water), whisking in sugar and vanilla until smooth.

Pour about two‑thirds of the vanilla batter into the prepared pan. Drop dollops of chocolate mixture or batter over the top, then gently swirl through with a knife or skewer to create a marble pattern. Finish with remaining vanilla batter and swirl again lightly.

Bake in preheated oven for about 35–40 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Avoid overbaking to preserve moistness.

Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.