1. Preheat the oven to 350 degrees F. Line a 9x13 inch baking dish with parchment paper, then butter/spray with cooking spray.2. In a large mixing bowl, beat together the eggs, buttermilk, greek yogurt, canola oil, and vanilla until smooth. Add the flour, sugar, unsweetened cocoa powder, baking powder, baking soda, and salt, mix until there are no longer any clumps of flour. Add the hot coffee and mix until combined. Batter should be pourable, but not super thin. Stir in the chocolate chips, if using.3. Pour the batter into the prepared baking dish, bake 20 to 25 minutes, until the top is just set and no longer wiggly in the center. Remove and let cool completely before frosting.4. To make the buttercream, add the butter and powdered sugar to the bowl of a stand mixer (or use a hand-held mixer). Beat the butter and powdered sugar together until the butter is light and fluffy, about 4 minutes. Add the melted milk chocolate and vanilla and beat (scraping down the sides as needed) another 2 minutes or until there are no streaks of white. Add 2 tablespoons of the heavy cream and whip the frosting for 2-4 minutes more or until light and fluffy. If desired add the remaining heavy cream and whip until combined. 5. Frost the cake and decorate as desired.