Preheat the oven to 350℉. Grease a 10-inch Bundt pan and dust with cocoa powder. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt.
In a large bowl, beat the butter and sugar together on medium-high until light and fluffy, about 4-5 minutes. Add the eggs in one at a time, mixing until each is incorporated.
Alternate adding the flour mixture and buttermilk, starting and ending with flour. Add the sour cream and vanilla and mix until just combined.
Pour the batter into the prepared pan. Bake for 65-70 minutes, or until a toothpick comes out clean from the center. You may also check for doneness using a thermometer. It will be done baking when the internal temp reaches 210℉.
Let the cake cool in the pan for 15-20 minutes before inverting onto a cooling rack to finish cooling.