Cut the butter into 4 pieces and chill. Preheat the oven to 180 degrees. Grease 4 muffin tins and sprinkle with flour.
Mix white sugar, 8 teaspoons flour, 4 teaspoons brown sugar.
Set aside 2 tbsp the resulting mixture into a small bowl and stir with the remaining 4 teaspoons of brown sugar and cinnamon (for steisel).
Add 1/3 cup flour, baking powder, baking soda, and salt to the rest of the mixture (no cinnamon).
Put butter on top, mix quickly with a knife or blender until finely crumbled.
Add egg and sour cream, stir until smooth.
Transfer to 1 tbsp dough in each muffin mold, put 1 tablespoon on top. spoon streusel. Place the remaining dough on the streusel. Bake for 20-25 minutes, until golden brown.
Chill the cinnamon muffins in the tins for 10 minutes. Then remove the muffins and refrigerate for another 10 minutes. Sprinkle with powder when serving.