Adjust oven rack to middle position and heat oven to 350°F Spray standard muffin tin with nonstick vegetable cooking spray.
Whisk flour, baking powder, and salt in medium bowl until combined. Whisk egg in second medium bowl until well-combined and light-colored, about 20 seconds. Add sugar and whisk vigorously until thick and homogeneous, about 30 seconds; add melted butter in 2 or 3 steps, whisking to combine after each addition. Add sour cream in 2 steps, whisking just to combine.
Add frozen berries to dry ingredients and gently toss to combine. Add sour cream mixture and fold with rubber spatula until batter comes together and berries are evenly distributed, 25 to 30 seconds (small spots of flour may remain and batter will be thick). Do not overmix.
Use ice cream scoop or large spoon to drop batter into greased muffin tin. Bake until light golden brown and toothpick or skewer inserted into center of muffin comes out clean, 25 to 30 minutes, rotating pan from front to back halfway through baking time. Invert muffins onto wire rack, stand muffins upright, and cool 5 minutes.
While muffins are cooling, mix sugar and ground cinnamon in small bowl and melt butter in small saucepan. After baked muffins have cooled five minutes, working one at a time, dip tops of muffins in melted butter and then cinnamon-sugar. Set muffins upright on wire rack; serve.