Place the milk in the mug mix with vinegar, and let it rest for about a minute. This quick step creates a homemade buttermilk that keeps the mug cake tender and soft.
Add the dry ingredients-flour, cocoa powder, baking powder, baking soda, instant coffee, and salt and mix together.
Add the white and brown sugar, oil, and vanilla, and mix until the mixture turns into a smooth, thick batter. Mix in the chocolate chips or chopped chocolate.
If you want a caramel center, spoon a little into the middle of the batter and gently cover it up.
Wipe down the inside of the mug so nothing burns along the edges, then microwave on high for about 1 minute. Serve your warm mug cake with a scoop of ice cream and your favorite toppings, and enjoy immediately.