Preheat oven to 325° F.
Cream the butter and sugar in a large bowl of a stand mixer fitted with the paddle attachment on medium speed until light and fluffy, about 10-15 minutes.
Meanwhile, in a separate bowl or glass measuring cup, stir together the instant coffee granules and the warm water until the coffee granules are completely dissolved. Stir in the buttermilk and vanilla extract; set aside.
Also while the butter and sugar are creaming, use a separate bowl to sift together the flour, cocoa powder, salt and baking powder. Set aside.
Once the butter and sugar have finished creaming, scrape the sides and bottom of the bowl. With the mixer on low speed, add the eggs one at a time, beating well after each addition.
Scrape the sides and bottom of the bowl; mix on low speed for 5 more minutes.
While the mixer is on low, alternately add the flour mixture and the warm buttermilk mixture, beginning and ending with the flour mixture. Mix until combined, scraping the sides of the bowl if necessary.
Grease and flour a Bundt pan. Pour the batter into the pan, just until it is about 1-1 ½ inches from the top. Do not over-fill the pan. Tap the pan on the counter once or twice to release air bubbles.
Bake for 75-85 minutes, or until a toothpick inserted in the center of the cake comes out clean.
Cool in the pan for about 10-15 minutes, and then remove to a wire rack to cool completely.
Once the cake is cool, dust with powdered sugar just before serving.