Spray a large microwave-safe coffee mug with cooking spray, set aside.
In small bowl, combine protein powder, gluten-free baking mix, sugar substitute, cinnamon and baking soda. Stir together until combine well.
Mix in egg, lemon juice, water, and coconut oil until combined.
Pour into sprayed coffee mug and microwave 1 minute 15 seconds, or until cooked through. Microwave times may vary depending on your appliance.
Remove mug and invert onto plate, gently slide out cake. Let cool for 2 minutes.
Using serrated knife, carefully slice cake crosswise through center, remove top and set aside.
Put ½ of whipped topping on one cake piece, sprinkle with a few of the pistachios and chocolate chips. Add top layer of cake and add remaining whipped topping, sprinkle with remaining toppings including lemon zest, if desired.