In a mug whisk together the flax egg (ground flaxseed and water). Add the vegan butter and microwave for 30 seconds to melt the better. Stir to combine.
Add the brown sugar, vanilla, and vinegar. Stir until well combined.
Add the flour, baking powder, and salt to the mug. Stir until well combined. Stir in the chocolate chips.
Microwave the mug cookie on high for 1 minute and 30 seconds. See notes below for different microwave strengths and cooking time.
Let the mug (or mugs) sit in the microwave for a minute or two to cool. Then serve your mug cookie as is or add your favorite toppings such as chocolate chunks, a dollop of coconut whipped cream, or a drizzle of vegan chocolate syrup.
Notes
This mug cookie is amazing when freshly baked. However, it can keep for up to 2 to 5 days in the fridge.
microwave Cooking Tips
The power of your microwave has a big impact on the amount of cooking time required for your mug cookie. Most microwaves fall between 800 and 1200 watts. Here's how to make this mug cookie based on your microwave:
Cook for 60 Seconds — Many mug cookies will be done at this point, especially if your microwave falls within the average of 800 to 1200 watts.
Check for Doneness — If your cookie has a little dome, or you can press your finger gently on the top, and it springs back, your mug cookie is done.
If you need to Cook it Some More — If your mug cookie is not entirely done after 60 seconds, cook it for an additional 15 seconds. Recheck it and repeat until it's done.