In a microwave-safe mug, whisk together the flour, coconut sugar, and baking powder.
Stir in the milk until the batter is smooth, then gently fold in the diced peaches.
In a small bowl, mix the streusel ingredients (flour, coconut sugar, and melted butter) with a fork until crumbly.
Sprinkle the streusel evenly over the batter.
Microwave for 70 to 80 seconds, until the center is just set.
Let cool for a minute before eating, since it firms up as it rests.