Preheat oven to 350°F. Line the bottom of four 4-inch (7oz) ramekins with rounds of parchment. Grease the ramekins well. Set aside.

With a stand mixer or hand mixer, combine sugar, flour, cocoa powder, baking powder, baking soda, and salt. Add egg, milk, oil, and vanilla extract and beat for 2 minutes. Add hot coffee or hot water, and beat until just combined. The batter will be thin.

Divide evenly between four prepared ramekins (I used a little over ⅓ cup batter per ramekin). Bake for approximately 20 minutes, until the top springs back when you touch them, and a toothpick inserted into the center comes out clean. Transfer the ramekins to a wire rack to cool completely.

Run a thin knife around the edge of each ramekin and carefully remove each cake. With a serrated knife, gently cut the dome off the top of each cake. Discard or save the cake scraps for another use.

For the frosting: Using a deep bowl, with a hand mixer or stand mixer, beat powdered sugar and butter until smooth (starting the mixer low, then slowly increasing the speed). Add vanilla extract and 1 teaspoon of milk. Beat on medium speed for a minute or two, until light and fluffy. If the frosting appears too thick, add an additional teaspoon of milk.

Spread the frosting onto the top of each cake. You can leave the cakes in a single layer or stack them to make a double layer cake. Decorate the top with berries or sprinkles.