Whisk together all ingredients for dry mix, breaking up any clumps of brown sugar. Store in the fridge.
To make a muffin, place butter in a mug, and microwave for 20 seconds until melted.
Add 1/3 cup dry mix, milk and vanilla.
In a separate bowl, gently beat 1 egg. Add half of the egg to the mug and whisk everything to combine (discard rest of egg or save for later use).
Microwave for approximately a minute and a half, until muffin springs back when lightly pressed down on top.
If desired, before cooking stir in 1/8 cup frozen berries.
Add 15-30 seconds to the cooking time to compensate for the added moisture of the berries.