1. Preheat oven to 350°F and place two cupcake liners in a muffin pan

2. Whisk flour, cocoa powder, sugar, baking soda and salt in a bowl

3. Make a well and add oil, milk, sour cream and vanilla

4. Whisk until smooth batter forms

5. Divide batter evenly between cupcake liners filling about two thirds full

6. Bake 17 to 20 minutes until toothpick comes out clean

7. Cool cupcakes for at least 10 minutes then transfer to rack

8. Beat butter until creamy for frosting

9. Add powdered sugar, cocoa powder and salt and mix

10. Add remaining sugar gradually and mix until smooth

11. Add milk if needed then frost cooled cupcakes