About the Recipe
I am not the author of this recipe. It is from Tiegen (Half Baked Harvest)

Ingredients
Cake:
2 eggs
¾ cup buttermilk
¼ cup plain Greek yogurt
½ cup canola oil
1 tablespoon vanilla extract
1½ cups all-purpose flour
1½ cups granulated sugar
1 cup unsweetened cocoa powder
1½ teaspoons baking powder
1½ teaspoons baking soda
1 teaspoon salt
¾ cup hot black coffee
½ cup semi-sweet chocolate chips (optional) (Guittard recommended)
Milk Chocolate Frosting:
1 cup (2 sticks) salted butter, softened
1½ cups powdered sugar
8 ounces milk chocolate + 2 ounces dark chocolate, melted and cooled (Guittard recommended)
2 teaspoons vanilla extract
2-3 tablespoons heavy whipping cream
Preparation
1. Prepare the Cake:
Preheat oven to 350°F (175°C).
Line a 9×13 inch baking dish with parchment paper and grease with butter or cooking spray.
In a large mixing bowl, beat together eggs, buttermilk, Greek yogurt, canola oil, and vanilla until smooth.
Add flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix until no dry clumps remain.
Pour in hot coffee and mix until combined. The batter should be pourable but not too thin.
Stir in chocolate chips (if using).
Pour batter into the prepared baking dish.
Bake for 20-25 minutes, or until the top is just set and the center is no longer wiggly.
Let the cake cool completely before frosting.
2. Make the Frosting:
In a stand mixer or with a hand-held mixer, beat butter and powdered sugar for about 4 minutes until light and fluffy.
Add melted milk chocolate and vanilla, beating for another 2 minutes until smooth.
Add 2 tablespoons of heavy cream and whip for 2-4 minutes until fluffy. Add the remaining cream if needed for consistency.
3. Assemble:
Frost the cooled cake evenly.
Decorate as desired.