Preheat your oven to 350°F (175°C). Grease and flour a 10-inch Bundt pan, or use a non-stick Bundt pan with a light spray of cooking spray.

1. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
2. In a large mixing bowl, cream together the softened butter and sugar using an electric mixer until light and fluffy (about 3-4 minutes).
3. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
4. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
5. Divide the batter evenly into two bowls. In one bowl, mix in the melted semi-sweet chocolate until fully incorporated.
6. Starting with the vanilla batter, spoon alternating dollops of the chocolate batter into the Bundt pan. Continue layering the batters, then use a butter knife or skewer to gently swirl the batters together to create a marbled effect.
7. Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
8. Let the cake cool in the pan for 10-15 minutes before carefully inverting it onto a wire rack to cool completely.