Preheat oven to 350°F.

Spray an 8”x4” loaf pan with nonstick baking spray; set pan aside.

Using an electric mixer, cream together sugar and butter until light and fluffy (3-4 minutes on medium speed).

Add sour cream, eggs and vanilla extract; mix on low speed until well combined.

Add flour and baking soda; mix on low speed until well combined.

Coarsely chop the chocolate and melt using a double-boiler.

Transfer ⅓ of the cake batter into a separate bowl. Add melted chocolate; stir until well combined.

Pour ~½ of the plain batter into prepared baking pan. Next, add chocolate batter, but do not stir together. Pour remaining plain batter on top.

Using the handle of a wooden spoon, gently swirl the batters together in 4-5 strokes. Do not overblend.

Bake at 350°F for 65-70 minutes, or until a toothpick inserted into center of cake comes out mostly clean. (Note: Tent pan with foil after 30 minutes to prevent the top from burning.)

Let cake cool before removing from pan and slicing.