Place rack in center of the oven then preheat to 375 degrees. Coat a standard-size muffin tin with cooking spray then set aside.

Whisk together flour, sugar, baking powder, baking soda and salt in a medium bowl then set aside.

In a separate, smaller mixing bowl, whisk together egg, milk, vanilla and melted butter. Add wet ingredients to the bowl containing the dry ingredients then mix until just combined.

Mix streusel ingredients in a small bowl using a fork to mash the brown sugar, flour and cinnamon together with the butter until combined but still crumbly.

Add ½ tsp of the streusel mixture to the bottom of each muffin cup. Pour enough muffin batter over the streusel to fill each cup a little less than half full. Add an additional ½ tsp streusel mixture over the batter then finish by filling each cup with remaining muffin batter.

Use a toothpick or butter knife to lightly swirl the batter and distribute the streusel through the center of the muffin. Sprinkle all remaining streusel over the top of the muffins.

Bake 18–20 minutes or until a toothpick in the center comes out clean. Cool muffins for 5 minutes then remove from pan and transfer to a wire rack to cool completely.