Preheat oven to 350°F. Spray a 12-cup bundt pan generously with baking spray. *Note, it MUST be a generous greasing of baking spray that contains flour in the ingredients. Avocado oil, butter, or other greasing methods do not work consistently, but baking spray has never failed us. 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. The batter will be very thin.
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.