In a medium bowl, whisk together the flour, salt, baking powder, and lemon powder (if using). In a separate large bowl, mix together the egg, brown sugar, yogurt, and vegetable oil.
Add the dry ingredients and the milk (alternating) to the wet ingredients, mixing to combine with each addition.
Fold in the blueberries with a gentle hand.
Make the streusel: Combine the cinnamon, flour, and brown sugar in a small bowl. Add the melted butter and mix to form clumps.
Spoon the batter into muffin cups or a greased muffin pan. Top with the streusel. (In terms of ratios, my perfect blueberry muffin follows the rule of three: one-third berries, one-third batter, and one-third streusel topping. )
Bake for about 20 minutes (start checking at 15 minutes) and remove when muffins are golden brown and the berries are bubbling with juice. Let cool.