1. Preheat oven to 350°F (175°C). Grease a 9×5-inch loaf pan and line it with parchment paper.

2. In a microwave-safe bowl, melt the chopped semisweet chocolate in 20-second bursts, stirring between each until smooth. Set aside to cool.

3. In a large bowl, beat butter and sugar together until light and fluffy, about 3 minutes.

4. Add eggs one at a time, then stir in vanilla extract until creamy.

5. Whisk together flour, baking powder, and salt in a separate bowl.

6. Add dry ingredients to butter mixture, alternating with milk. Begin and end with flour. Mix just until combined.

7. Divide the batter in half. Stir melted chocolate (and cocoa powder, if using) into one portion.

8. Spoon alternate dollops of vanilla and chocolate batter into prepared pan. Use a knife to gently swirl through the batter.

9. Bake for 45–55 minutes, or until a toothpick inserted in the center comes out clean.

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