Preheat oven to 350F. Spray a 9X13 pan with non stick baking spray. Place a layer of parchment paper on the bottom. Spray again.
In a mixing bowl, sift the flour, cocoa powder, baking soda, baking powder, salt, sugar, and brown sugar together.
Heat the hot water in a saucepan until steaming. Pour hot water in a glass measuring cup. Stir in the instant espresso.
In a seperate bowl, whisk the buttermilk, eggs, egg yolk, vanilla, and oil. Slowly mix in the espresso. Using a stand mixer, slowly pour in the wet ingredients into the dry.
Once combined, scrape the bowl and mix for 20-30 seconds. Take off the mixer. Fold the mini chocolate chips in with a rubber spatula.
Pour the cake batter into the pan. Smooth with the spatula. Bake for 23-28 minutes until a toothpick inserted comes out clean. Let sit in pan for 10 minutes. Transfer to a cooling rack and allow to cool completely before frosting.