Preheat your oven to 325 degrees Fahrenheit.
Grease a medium bunt pan with butter (8 1/2 inch or 9 inch) and set aside.
In a large mixing bowl with a hand mixer, or in the bowl of your stand mixer, mix the butter and sugar together on medium high speed for several minutes until light and fluffy in texture and pale in colour.
Add the vanilla extract and mix until incorporated.
Add in one egg at a time, mixing on high speed for about one minute and scraping down the bowl after each addition. Mix in the sour cream.
In 2 medium bowls, divide the flour in half (1 1/8 cup in each bowl). Add the cocoa powder to one bowl and divide the salt between the 2 bowls (1/4 teaspoon to each bowl). Whisk the flour mixtures in the 2 bowls separately with a fork, just to combine.
Pour half the butter, sugar and egg mixture into each of the bowls with the separate flour mixtures. You should have one with cocoa powder, and one without.
Fold the dry and wet ingredients together gently in each bowl separately, just until no streaks of flour remain.
Dollop spoonfuls of batter into the bundt pan, alternating colours. Use a wooden skewer or butter knife to create a swirl pattern in the batter before baking.
Bake at 325 degrees Fahrenheit for about 65 minutes or until a toothpick inserted into the middle of one side of the cake comes out clean.
Cool for about 10 minutes in the pan before inverting onto a cooling rack (remove the cake pan) to cool completely to room temperature before slicing and serving.