Beat the room temperature butter and oil together until completely homogenous. Stream in the granulated sugar and beat until combined. Add the eggs and vanilla extract and mix until combined.
Add half of the dry ingredients and mix until just about incoporated. Add half of the buttermilk and mix until just about incorporated. Repeat this with the remaining dry ingredients and buttermilk. Make the chocolate ganache and set aside.
Spoon the batter into the cupcake tin until it's 2/3 of the way full. Bake for 18-20 minutes until a toothpick comes out clean.
While the cupcakes are baking, make the buttercream. Beat together the butter, powdered sugar, cocoa powder, salt, milk/heavy cream, and vanilla extract until combined and smooth.
Once the cupcakes are completely cool, fill with the ganache, frost and enjoy!