Make the cake
Prepare a 9" cake pan (I used a springform pan) with cake spray or butter and parchment.
Preheat the oven to 350F.
Whisk together the dry ingredients - your flour, cocoa powder, baking soda, and salt.
In the bowl of a stand mixer, mix oil, coffee, vinegar, water and sugar.
Add the wet ingredients and mix to combine. Pour into your pan and bake until the top is springy, about 35-40 minutes.
After 5 minutes out of the oven, cool the cakes on a wire rack - or better yet, freeze them wrapped in saran wrap for a few hours - before frosting.
Make the frosting
Prepare a water bath -- fill a medium pot with about 3 inches of water and bring to a low boil.
Place all your frosting ingredients in the bowl of a KitchenAid mixer (or a metal bowl if you don't have a KitchenAid). Afix a candy thermometer to your bowl. Place atop the water. Whisk the mixture constantly until it reads 160F -- the mixture will be bubbly and the sugar will have dissolved.
Cover your hand with a pothandle and transfer to your stand mixer with a whisk attachment. Mix on high for 4 minutes, until very fluffy with stiff peaks.
Spread onto your cooled cake and top with sprinkles.