Preheat the oven to 400ºF/200ºC. Line a 12-hole muffin pan with paper cases.
First, make the streusel topping: add all streusel ingredients except the melted butter to a bowl and mix. Add the melted butter and use your fingers to rub it all together into large crumbs. Set aside.
In a large mixing bowl, whisk the flour, baking powder, sugar, cinnamon and salt.
In a smaller bowl, whisk the eggs, milk, melted butter and vanilla together.
Stir wet mixture into the dry mixture and gently mix to combine.
Spoon the batter into the muffin cases to ¾ full. Sprinkle the tops of each one with the streusel mixture.
Bake for 20-25 mins until the cinnamon muffins are risen and golden. Allow to cool on a wire rack.