Preheat a pancake griddle to medium heat. Whisk together the pancake mix and water in a bowl.
Place the still-frozen strawberries into a blender, and pulse several times to break the berries up into small pieces. Mix the chopped strawberries and the frozen blueberries into the batter until the fruit is well distributed.
Grease the griddle with butter, and pour about 1/4 cup of batter per pancake onto the griddle. Allow the bottom side to brown and the batter to form air holes; sprinkle the uncooked side with brown sugar, if desired, and flip to brown the other side.