Preheat the oven to 350°F/180°C. Prepare a 6" baking pan by greasing with oil, or using a round cake pan parchment liner. Then, grab three bowls--one small, one medium, and one large.
In the small bowl, add the cocoa powder and pour the hot water over it. Use a fork to stir it into a thick chocolate paste. Let it cool slightly until it just warm to the touch.
In the medium bowl, combine the flour, baking soda, baking powder, and salt until they are mixed together.
In the large bowl, whisk the two types of sugar, oil, egg, and vanilla together until smooth (approximately 1 minute of whisking). Then, whisk in the buttermilk and cooled chocolate paste until they are well blended. Lastly, add the flour mixture and gently fold it in until the flour is just combined. The batter will seem runny and that is normal.
Pour the batter into your prepared cake pan. Bake for 25-30 minutes. You can test if the cake is ready by gently tapping the top, which should spring back once done. Remove the cake from the oven and let it cool for 10-15 minutes in the pan, before removing and setting on a wire rack to cool completely.
To make the frosting, start by whipping the butter and salt together in a mixer until combined. Then add the remaining ingredients. Continue to beat on medium-high speed until the frosting is light and fluffy (2-4 minutes, depending on your mixer).
Make sure the cake is completely cool before frosting. You can use either an offset spatula to spread an even layer across the top, or pipe rosettes using a star tip (I used Wilton's M1 tip). Garnish with freeze dried raspberries if desired. Enjoy!