Preheat oven to 400 degrees. Line a muffin tin with muffin liners.
In a medium bowl, mix together the all purpose flour, baking soda, baking powder, cinnamon, and salt.
Mix the melted butter and brown sugar together until fully combined. Add the eggs, sourdough starter, and apple sauce. Mix until combined.
Add the dry ingredients to the wet ingredients and mix until just combined. Try not to overmix, as that will cause the muffins to be tough.
In another small bowl, add the streusel ingredients and mix them to combine them.
Scoop the batter into the muffin tins and top each with about a tablespoon of the streusel mixture. Put the muffin tin in the oven and bake for 22-25 minutes, or until a toothpick comes out clean.