Preheat oven to 350 degrees F. Line an 8x8-inch glass or ceramic baking dish with parchment paper and spray sides and bottom. If using a metal baking pan, heat oven to 325.
Mix flour, sugar, cocoa powder, baking powder, baking soda, and salt in bowl of stand mixer.
Add in egg, milk, oil, and vanilla and mix on low until incorporated. When mixed in increase speed to medium and mix for 2 minutes.
Add in boiling water and gently stir- batter will be very thin.
Pour batter into prepared dish. Bake for 35-40 minutes, or until an inserted toothpick is removed clean.
Cool for 10 minutes in pan, then continue cooling on a rack until room temperature.
While cake is cooling, make the frosting.
In a medium microwave-safe bowl, heat the chocolate chips in the microwave at 50% power in 30-second increments, stirring after each heating, until they are fully melted.
Add the sour cream and vanilla and stir by hand until frosting is well combined and fluffier.
Spread over cooled cake.
