01 -
Preheat the oven to 350°F (175°C) for a glass or ceramic dish, or 325°F (163°C) for a metal pan. Line an 8×8-inch baking dish with parchment paper and lightly grease the sides and bottom.
02 -
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and kosher salt.
03 -
Add the egg, milk, vegetable oil, and vanilla extract to the dry mixture. Mix on low speed until combined, then increase to medium speed and mix for 2 minutes.
04 -
Stir in the warm water or coffee until fully incorporated. The batter will be thin.
05 -
Pour the batter into the prepared baking dish and bake for 35-40 minutes or until a toothpick inserted in the center comes out with a few crumbs but no wet batter.
06 -
Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
07 -
In a microwave-safe bowl, heat the chocolate chips at 50% power in 30-second intervals, stirring after each, until fully melted.
08 -
Stir in the sour cream and vanilla extract into the melted chocolate until smooth and fluffy.
09 -
Spread the frosting evenly over the cooled cake.