Preheat oven to 375 degrees and line muffin tin with muffin liners or grease really well.
In a large bowl, gently whisk flour, brown sugar, baking powder, baking soda, cinnamon, and salt together.
In a smaller bowl, whisk milk, oil, eggs and vanilla until completely combined. Make a well in the center of dry mixture and pour in liquid. Mix until just combined, being cautious to not overmix the batter.
Scoop the batter evenly into muffin liners--this about ¼ cup for each muffin.
For the crumb topping, combine cinnamon, sugar brown sugar, flour, and salt together. Mix in the butter (or coconut oil) until just crumbly, using 2 forks or your fingers. Sprinkle ½ to 1 tablespoon of the crumble over each muffin.
Bake muffins for 15-17 minutes or until a toothpick inserted into the center come out dry.
Cool on a rack for 10 minutes and then remove muffins from pan. Allow to cool fully before storing.
To keep these muffins Egg-Free, use or ⅓ cup water mixed with 2 tablespoons ground flaxseed in place of the two eggs.
For dairy-free muffins, use almond milk in place of the milk and solid coconut oil in place of the butter in the crumb topping.
Feel free to skip the crumb topping.
If you are NOT gluten-free, go ahead and make these Cinnamon Streusel Muffins with whole wheat or unbleached flour (it is an equal substitution).
Store muffins in an air-tight container for 3 days at room temperature and up to 5 days in the refrigerator.