Preheat oven to 350 degrees F. In a bowl, mix together brown sugar, flour, cinnamon and salt for the streusel topping. Cut in cold butter pieces using a fork or your fingers, until small crumbs remain. Set aside.
In a smaller bowl, mix together flour, baking powder, cinnamon, baking soda and salt. In a larger bowl, whisk together eggs and sugar until smooth, then add in yogurt. Whisk for a minute or two (it will start out lumpy!) until combined and light in color, then whisk in cooled butter. Fold in the dry ingredients just until combined. Do not overmix!
Line a muffin tin with liners or spray well with nonstick spray. Fill the muffin tins almost halfway full with batter, then sprinkle with some of the streusel topping. Add another few tablespoons of batter on top, making sure to slightly press down so the top and bottom batters meet (or else the muffins will sort of separate easily). Top with another handful of the streusel topping.
Bake for 18-20 minutes, or until golden on top. Serve hot with butter!