For streusel topping:
Preheat oven to 375°F. Line a muffin pan with paper liners.
Sift the first five ingredients together in a medium bowl. In a separate bowl, whisk the egg with the cooled butter, vanilla, and milk. Pour the wet ingredients into the dry and stir until mostly combined. Fold in the apple and pecans until just combined (do not overmix). Fill each liner two-thirds full with prepared batter.
Combine all of the ingredients for the topping in a small bowl and gently knead together with fingertips. Sprinkle evenly over muffins. Bake for 20-25 minutes or until tester inserted comes out with moist crumbs attached. Transfer to a wire rack to cool slightly. Serve warm.