Preheat oven to 350 degrees F. Prepare muffin tins with 24 paper liners.
In large mixing bowl whisk together cocoa powder and baking soda. Carefully pour in boiling water and whisk until well blended; allow to cool for 5 minutes.
Meanwhile, in another large mixing bowl add the melted butter, vegetable oil, sugar, vanilla and salt and beat until well combined. Add eggs and eggs yolks, mixing after each addition, just until combined.
Stir in cocoa mixture and then milk. Slowly mix in flour and stir until combined.
Divide batter evenly among the 24 muffin cups, filling each cup about 2/3 full.
Bake for 18-20 minutes or until toothpick inserted into center comes out clean. Remove from oven and let cool for a few minutes before transferring to wire rack to cool.
For the Frosting:
Beat butter for two minutes. Add vanilla extract, cocoa powder and milk and beat again until smooth. Slowly add powdered sugar until you reach your desired frosting consistency.
Pipe frosting onto cooled cupcakes and top with sprinkles!
To make ahead: The cupcakes and the chocolate buttercream frosting can both be made ahead of time. Allow the cupcakes to cool completely, and then store them in a ziplock freezer bag. Freeze for up to one month. I like to frost the cupcakes when they are frozen because they're easier to frost.To freeze: Freeze your cupcakes before frosting them. Frosting cupcakes is so much easier when they are frozen. Cupcakes thaw quickly so even if you want to serve them an hour or two after frosting, it shouldn't be an issue. You could also freeze frosted cupcakes, but I suggest flash-freezing them on a tray for 30 minutes (to help the frosting keep its shape) before placing them in a freezer safe container.