Preheat the oven to 375 degrees Fahrenheit. Line a 12-cup muffin pan with paper liners or lightly grease each cup.

Prepare the streusel topping by adding the lemon zest and granulated sugar to a food processor. Pulse until the lemon zest is finely incorporated into the sugar.

Transfer the lemon sugar to a small bowl. Add the all-purpose flour and cold butter. Use a fork, pastry cutter, or your fingertips to work the butter into the dry ingredients until coarse crumbs form. Set aside.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt.

In a separate bowl, whisk together the milk, vegetable oil, egg, fresh lemon juice, lemon zest, and vanilla extract until well combined.

Pour the wet ingredients into the dry ingredients. Stir gently until just combined. Do not overmix; a few small lumps are perfectly fine.

Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.

Sprinkle the lemon streusel evenly over the tops of each muffin.

Bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.