Preheat oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan.
In a large bowl, cream together butter and 1 cup sugar until light and fluffy. Add eggs one at a time, mixing well after each. Stir in vanilla and sour cream.
In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually add dry ingredients to wet ingredients until just combined.
In another bowl, beat cream cheese, 1/4 cup sugar, egg, and vanilla until smooth.
Pour half of the chocolate batter into the prepared pan. Add cream cheese mixture, then top with remaining chocolate batter. Swirl gently with a knife.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Cool in pan for 15 minutes, then transfer to a wire rack to cool completely.