Line your cupcake pans with liners and set aside (I made 24 minis & 10 full size).
Preheat oven to 350 degrees.
In a large bowl, mix together the flour, cocoa powder, sugar, baking soda, baking powder and salt with a whisk.
Add the eggs, oil, vanilla and milk and mix until combined. It will be very thick at this point.
I switched to a wooden spoon and added the hot water. Mix until fully incorporated. Now batter will be thin.
Fill the liners about 3/4 full and bake for full size - 12-15 minutes, minis - around 10 minutes until a toothpick inserted in the center comes out clean.
Cool completely before you start decorating.
For the Buttercream
Make sure you start with softened butter. I used my stand mixer but a hand mixer will work fine, too.
With the paddle attachment, whip the butter until it starts to get smooth.
Add the powdered sugar, one cup at a time, on medium speed for 1 to 2 minutes.
Add the heavy cream and vanilla extract and mix on high for 3 to 4 minutes.
If you use UNsalted butter, you'll want to add a pinch of salt. I started with salted butter so there's no need.
Once cooled, core out the center of your cupcakes. I used some type of apple corer to do this (I'm not exactly sure what it's intended use is, I just found it in my drawer & it worked perfectly).
Using a piping bag (or ziploc bag with a tip), fill the center of the cupcake with some frosting then replace the top. Frost the top with the buttercream - I did a simple swirl. Then sprinkle away. Some I smushed into sprinkles, some I actually spinkled.