Preheat your oven to 350°F (175°C). Grease a 10–12 cup bundt pan thoroughly and dust lightly with flour, tapping out the excess.

In a large bowl, cream the butter and sugar together until pale and fluffy, about 3–4 minutes with an electric mixer.

Add the eggs one at a time, beating briefly after each addition to keep the batter smooth.

Mix in the sour cream and vanilla until combined.

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

Add the dry ingredients to the wet in two additions, mixing just until there are no streaks of flour.

Divide the batter evenly between two bowls. Stir the melted, cooled chocolate and sifted cocoa into one bowl until uniform.

Spoon alternating dollops of vanilla and chocolate batter into the prepared pan. Run a butter knife through the batter to create a marble pattern.

Bake for 50–60 minutes, starting to check at 45 minutes by inserting a toothpick into the deepest part.

Cool the cake in the pan for 10 minutes. Then, invert onto a wire rack to cool completely.

Once cooled, drizzle with your favorite ganache or chocolate glaze.