Preheat oven to 375 degrees. Grease 12 muffin cups, or put liners in.
In a small bowl, stir together flour and brown sugar. Cut in butter until the mixture resembles fine breadcrumbs. Stir in pecans and set aside.
In a medium bowl, stir together all of the dry ingredients.
In another bowl, melt butter. Whisk in egg and milk.
Add the wet ingredients to the dry ingredients and stir until just moistened. Fold in the peaches.
Divide the mixture evenly between the muffin cups. Top with streusel.
Bake 18-20 minutes, or until a toothpick inserted in the center of the center muffin comes out clean.