Brownie in a Mug from celebrating sweets.com

3 tablespoons granulated sugar
 3 tablespoons all purpose flour
1 ½ tablespoons unsweetened cocoa powder
Pinch salt
 2 tablespoons chocolate chips, plus more for topping
 1 ½ tablespoons vegetable oil, melted coconut oil or melted butter
3 tablespoons milk
 ¼ teaspoon vanilla extract
 Ice cream, whipped cream, powdered sugar, berries for garnish
Instructions
Place sugar, flour, cocoa powder and salt in a mug. Use a small whisk or fork, and stir until combined, and free of lumps. Stir in chocolate chips. Add oil, milk and vanilla, stir with a spoon or small rubber spatula until just combined (try not to over mix it). 

Sprinkle a few additional chocolate chips over the top and cook in the microwave for about 90 seconds (microwave times may vary). Eat immediately with ice cream, whipped cream, powdered sugar, or berries.

Notes
I have a used a 12oz and 16oz mug for this recipe.
Although this is a single serving dessert, you can easily split this between 2 adults.