• Preheat the oven to 190ºC and line the muffin tin with paper liners
  

To make the streusel topping

• In a small bowl, mix the flour, brown sugar and baking powder until well combined.
• Add the butter (cut in small cubes) and work it through with your fingertips until the mixture is roughly crumbled and no large pieces of butter are visible.
• Set the bowl aside, until use. 

To make the muffin batter

• Using an electric hand mixer, beat the butter, sugar and lemon zest until the mixture is light and creamy. (you an also whisk by hand)
• Add in the egg and mix well *
• In a separate bowl, sift the flour, baking powder and salt.
• Add half of this dry mixture, to the butter mixture, and mix well using a spatula.
• Add the milk, and mix well.
• Add the remaining half of the dry mixture until smooth.
  
• Gently stir in the blueberries
  

Baking

• Fill in the muffin mixture into the prepared muffin tray. 
•  Generously sprinkle the streusel topping on top of the mixture, and bake for 35 minutes, on until an inserted toothpick comes out clean
• Cool the muffins in the tray for about 15 minutes and then cool completely on the cooling rack, for about 30 minutes.