Combine flour,sugar,salt,baking powder,baking soda,and cinnamon together in a large bowl. Using a pastry blender, work the butter into the dry ingredients until you have coarse crumbs.
Using an electric mixer on low speed, add the sour cream,vanilla and beaten egg. Mix together until the ingredients are just incorporated and batter is smooth. Add the diced apples, stir into the batter until just incorporated.
Fill muffin tins halfway full with batter. Press the streusel into clumps and sprinkle over batter. (it may look like you have more topping than you need , just keep adding it). Sprinkle chopped pecans over streusel,if desired.
Bake at 350 degrees for 30 minutes, rotating the pans halfway through baking.