Preheat the oven to 350°F (175°C). Grease and flour a 10-inch bundt pan, tapping out excess flour.
In a medium bowl, sift together the flour, baking powder, baking soda, salt, and cocoa powder. Set aside.
In a small bowl, combine the boiling water and cocoa powder, whisking until smooth and well combined. Let cool slightly.
In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together on medium speed until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Alternately add the dry flour mixture and buttermilk to the butter mixture, beginning and ending with the flour mixture. Mix each addition just until combined.
Slowly add the cocoa and boiling water mixture to the batter, mixing on low speed until fully incorporated. The batter will be thin.
Pour the batter evenly into the prepared bundt pan and smooth the top with a spatula.
Bake in the preheated oven for 45-50 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 for 15 minutes.
Invert the cake onto a wire rack and let it cool completely before serving.