Preheat the oven to 375°F (190°C)
Line a 12-cup muffin tin with paper liners or lightly grease it.
Blend the lemon zest with the sugar in a food processor until the zest is finely ground.
Transfer the mixture to a small bowl.
Add the remaining streusel ingredients to the zest mixture and use a fork to work the butter in until crumbly. Set aside.
Stir the flour, sugar, baking powder, and salt together in a large bowl.
Whisk the milk, oil, egg, lemon juice, lemon zest, and vanilla in another bowl.
Pour that mixture into the dry ingredients. Stir just until combined; do not overmix.
Divide batter evenly among the prepared muffin cups, filling about ¾ full.
Sprinkle streusel evenly over the tops of the muffins.
Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean.
Cool for 5 minutes in the pan, then transfer to a wire rack.