Preheat the oven to 350°F (175°C). Grease and flour a 10-inch bundt pan thoroughly.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and unsweetened cocoa powder. Set aside.
In a large mixing bowl, beat the softened unsalted butter and granulated sugar together on medium speed until light and fluffy, about 3 to 4 minutes.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Alternately add the dry ingredients and buttermilk to the butter mixture, beginning and ending with the dry ingredients. Mix each addition just until incorporated.
Stir in the sour cream until combined.
Carefully add the hot water to the batter and mix on low speed until smooth. The batter will be thin.
Pour the batter evenly into the prepared bundt pan.
Bake in the preheated oven for 45 to 50 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan on a wire rack for 15 minutes.
Invert the bundt pan onto a serving plate and gently lift the pan off the cake. Let the cake cool completely before slicing and serving.