Mix all of the dry ingredients together in a bowl.
Melt the butter in the microwave and stir into the dry ingredients with a fork until a crumble forms.
Cover and refrigerate until ready to use.
To make fruit muffins
In a medium bowl, whisk together the flour, sugar, baking powder, baking soda and salt.
Add the softened butter and mix it in until the mixture resembles a coarse crumble. Set aside.
In a separate bowl, whisk together the eggs, vanilla, and sour cream (or buttermilk).
Add the wet ingredients to the bowl with the dry ingredients and stir just to combine.
Toss the frozen berries (or frozen fruit) in the extra tablespoon (8 grams) of flour, then fold them in gently with a wooden spoon or spatula.
For a “regular” muffin top: divide the batter between 10 muffin paper-lined wells of a muffin pan (~70 grams batter per paper), sprinkle with streusel, and bake at 350°F (175 °C) for about 25 minutes. For a “domed” muffin top: divide the batter between 10 muffin paper-lined wells of a muffin pan (~70 grams batter per paper), sprinkle with streusel topping and bake at 425°F (220 °C) for 8 minutes, then drop temperature setting to 350°F (175 °C) for the last 15–17 minutes. For a “puffy” muffin top: refrigerate the batter overnight, then scoop the batter into 10 muffin paper-lined wells of a muffin pan (~70 grams batter per paper), sprinkle with streusel topping, and bake them at 350°F (175 °C) for about 25 minutes.
Notes
If you want larger muffins, scoop more batter per muffin cup and make 8 muffins instead of 10! Try a large disher to make the task easier!