Preheat the oven to 425°F and line the muffin tin with liners or spray with baking spray.
Make the streusel topping by combining the flour, brown sugar, and cinnamon. Then pour in the cooled melted butter and mix until crumbly. Set aside.
Sift together flour, baking soda, baking powder, cinnamon, and nutmeg in a bowl. Set aside.
In a separate mixing bowl, whisk together the eggs, oil, milk, vanilla, sugar, shredded carrots, and walnuts until well combined.
Gently mix in the dry ingredients with the wet ingredients and mix just enough to combine.
Using a measuring cup or cookie scoop, divide the batter among the 12 muffin liners, filling just below the top. Divide the streusel topping mixture among the 12 muffins, adding onto the muffin batter.
Bake at 425°F for 5 minutes, then reduce the heat to 350F and continue baking for 12-15 minutes, or until the toothpick comes out clean from the center of the muffin.
Remove the muffin tin from the oven and cool for 5 minutes. Transfer the muffins to a cooling rack and continue cooling or enjoy the muffins warm.