Blackberry streusel muffins

1 1/2 cups flour
1/3 cup sugar
2 tsp baking powder
1/4 tsp salt
1 egg
3/4 cup milk
1/4 cup oil
1 tsp lemon juice
1/2 tsp vanilla
2 cups frozen blackberries

Streusel topping (recipe below)

Grease 12 muffin cups, or line with paper cups.

Mix dry ingredients in a bowl. In a seperate bowl beat the egg, then add milk, oil, lemon juice and vanilla, mixing well. Pour wet ingredients into dry ingredients' bowl. Stir until just moistened - lumpy batter is good. Stir in blackberries.

Spoon batter into muffin cups until 2/3 full. Sprinkle streusel topping on each muffin. Bake in preheated oven at 400 degrees for 18 minutes or until toothpick inserted in center comes out clean.

Streusel topping:
3 Tbsp flour + 3 Tbsp brown sugar + 1/2 tsp cinnamon + 2 Tbsp butter + 2 Tbsp chopped nuts

Mix until crumbly.