Mix 1½ cups flour, baking powder, baking soda, 3 teaspoons cinnamon and ¼ easpoon salt in a large mixing bowl.
Mix in 4 tablespoons melted butter, ½ cup brown sugar, ½ cup granulated sugar, vanilla, milk and egg until smooth.
Add the sourdough starter and mix. Set aside for 15 minutes.
In a separate bowl, mix together by hand, the ingredients for the topping. The topping will be clumpy.
Place muffin liners in your muffin tin. Using a large scoop, add the batter equally to each cup.
Ass the topping to the top of each muffin, fully covering the batter.
Bake the muffins in the preheated over for about 15 minutes, until the muffins are cooked through. Test by sticking a toothpick into the center of a muffin. It should come out without any liquid on it. If not, bake a bit longer.