Preheat the oven at 350 F for 10mins.

Mix butter and sugar.

When they are mixed well, add egg, vanilla extract and milk. Mix well.

Now add the dry ingredients-flours, salt & baking powder a little at a time.

Mix till it forms a uniform mixture.

Now add zest and blueberries.

Fold them with a spoon.

Line a muffin tin with muffin cups.

Fill them 3/4th with the mix.

In a separate bowl, mix the ingredients of the topping.

Spread a little mixture on top of each muffin.

Bake in the preheated oven at 350F for about 15mins or till a toothpick inserted in the center comes out clean.