Preheat the oven to 350 degrees F, line a 12-cup cupcake pan. In a medium bowl combine the flour, cocoa powder, baking soda, and salt. Whisk to combine. Set aside.
In a large bowl combine the oil, sugar, egg, and vanilla. Whisk to combine until light and creamy. Slowly mix in milk.
Add the dry ingredients into the wet ingredients, mix well until completely combined. Use a portion scoop to fill each cupcake liner 2/3rd's full. Bake at 350 for 18-20 minutes or until a toothpick inserted comes out clean. Allow to cool completely before frosting.
For the buttercream:
In the bowl of a stand mixer fitted with the paddle attachment or using an electric hand mixer, cream butter until light and fluffy, 3-4 minutes. Add in the powdered sugar, one cup at a time. Mix thoroughly each time. Mix in the vanilla extract and the heavy cream, 1 tbsp at a time until desired consistency is reached.
Frost cooled cupcakes and top with chocolate curls or a chocolate chunk.