Preheat oven to 350°F. Spray a 12-cup bundt pan generously with baking spray.
Combine the coffee and cocoa powder in a microwave-safe bowl. Microwave for 1½ - 2 minutes, whisking well every 30 seconds. Set aside to cool.
1 ¼ cups brewed coffee, ¾ cup cocoa powder
In a stand mixer fitted with a paddle attachment (or a bowl with a hand mixer), combine the sugar, kosher salt, baking soda, eggs, and egg yolk on low speed for 1 minute. Scrape sides of bowl with rubber spatula.
2 ¼ cups granulated sugar, 1 ¼ teaspoon kosher salt, 2 ½ teaspoon baking soda, 2 eggs + 1 egg yolk
Add buttermilk, oil, and vanilla and mix on low speed for 1 minute. Scrape sides of bowl.
1 ¼ cups buttermilk, 1 cup vegetable oil, 1 ½ teaspoon vanilla
Add flour and mix on medium for 2 minutes. Scrape sides of bowl.
2 ½ cups all-purpose flour
Add the mostly cooled coffee-cocoa powder mixture (if it's too hot it will make the eggs lumpy) and mix on medium speed for 3 minutes. Scrape sides of bowl.
Pour batter into the prepared bundt pan. Bake on the center rack for 50-60 minutes, or until a toothpick inserted in the middle comes out clean or with a few moist crumbs. Allow cake to cool for 15 minutes in the pan. Invert the warm cake on a cooling rack and allow to cool fully.