Step 1

Preheat oven to 350°F. Grease an 8-inch or 9-inch square or round cake pan or line with parchment. Set aside.

Step 2

In a large mixing bowl, whisk together the flour, sugar, baking soda, baking powder, and salt. Make a well in the center and add the egg, oil, and buttermilk. Whisk to combine.

Step 3

Add the hot water and whisk again until smooth. Add the vanilla extract; whisk to combine. At this point you should have a very thin, smooth batter.

Step 4

Pour the batter into the prepared pan. Tap the pan gently on the counter a few times to release any big air bubbles. Bake for 25-30 minutes, or until a toothpick inserted in the center of the cake comes out clean.

Step 5

Let the cake cool completely before frosting.

Step 6

In a large bowl, beat the butter, buttermilk (or cream), and vanilla extract with an electric mixer on medium-high speed until smooth, about 1 minute. Reduce the speed to medium-low, and gradually add the powdered sugar. Beat until smooth, about 4 minutes. Increase the speed to medium-high and beat until the frosting is light and fluffy, about 4 more minutes.

Step 7

Spread the frosting on the cooled cake. Garnish with sprinkles, if desired.