Preheat oven to 350°F. Spray a 10-inch bundt pan generously with baking spray. Use a clean paper towel to wipe off any excess baking spray from the top of the pan.
Baking spray
Microwave coffee and cocoa powder in a microwave-safe bowl for 1 ½-2 minutes, whisking well every 30 seconds. Set aside to cool.
⅔ cups brewed coffee, ⅓ cup cocoa powder
In a stand mixer fitted with a paddle attachment or in a bowl with a hand mixer, combine the sugar, kosher salt, baking soda, egg, and egg yolk on low speed for 1 minute. Scrape sides of bowl with rubber spatula.
1 cups sugar, ½ teaspoon kosher salt, 1¼ teaspoon baking soda, 1 eggs + 1 egg yolk
Add buttermilk, oil, and vanilla and mix on low speed for 1 minute. Scrape sides of bowl.
⅔ cups buttermilk, ½ cup vegetable oil, 1 teaspoon vanilla
Add flour and mix on medium for 2 minutes. Scrape sides of bowl.
1¼ cups flour
Add cooled coffee-cocoa powder mixture and mix on medium speed for 3 minutes. Scrape sides of bowl.
Pour batter into prepared mini bundt pan. For our pan it was just over 2 tablespoons of batter (or 1 ½ scoops from a 1½ tablespoon batter scoop (affiliate)). Bake on the center rack for 14-15 minutes, or until a toothpick inserted in the middle comes out clean. Allow cakes to cool for 10 minutes in the pan. Invert the warm cakes on a cooling rack and allow to cool fully.
Wash mini cake pan and repeat with remaining batter.