Put the flour, sugar, cocoa, pecans, baking powder and salt in a 12-ounce microwave-safe mug. Combine well with a fork. Add the milk, vegetable oil and vanilla and mix well until smooth. Add the chocolate chips and caramel pieces and submerge them in the batter. Wipe the mug to clean.
Microwave on high for 90 seconds. Let cool for 1 to 2 minutes in the microwave before topping with the marshmallow creme (or vanilla ice cream) and drizzling with caramel sauce.