Preheat your oven to 350°F (175°C).
Prepare a muffin tin with paper liners or grease it with butter.
In a bowl, whisk together flour, sugar, baking powder, salt, and cinnamon.
Cream the butter until it is nice and creamy. Gradually add the brown sugar.
Mix the eggs and vanilla into the butter-sugar mixture until smooth.
Blend the dry ingredients into the wet mixture, alternating with the milk.
Fill each muffin cup about 3/4 full with batter.
Make the streusel topping by combining cinnamon, brown sugar, and flour. Sprinkle this over the batter.
Bake for 18-20 minutes or until a toothpick comes out clean.
Cool the muffins in the tin for a few minutes before moving them to a wire rack.
