01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - Combine chopped nuts, brown sugar, flour, and cinnamon in a small bowl. Cut in cold butter using a fork or pastry cutter until mixture forms coarse crumbs. Set aside.
03 - In a large bowl, whisk together flour, granulated sugar, baking powder, baking soda, salt, and cinnamon.
04 - In a separate bowl, whisk together eggs, milk, melted butter, and vanilla extract until smooth.
05 - Pour wet ingredients into dry mixture and stir gently just until combined. Avoid overmixing; batter should remain slightly lumpy.
06 - Divide batter evenly into prepared muffin cups, filling each about two-thirds full.
07 - Sprinkle streusel topping generously over each portioned batter.
08 - Bake muffins for 16 to 18 minutes or until a toothpick inserted in the center comes out clean.
09 - Let muffins cool in the pan for 5 minutes before transferring to a wire rack to cool completely.