Preheat your oven to 350°F (175°C) and grease a donut pan.
In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon.
In another bowl, combine the milk, vegetable oil, eggs, and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Spoon the batter into the prepared donut pan, filling each cavity about 2/3 full.
In a separate bowl, mix the crumb topping ingredients: flour, brown sugar, cinnamon, and melted butter until crumbly.
Sprinkle the crumb topping generously over the batter in the donut pan.
Bake for 12-15 minutes or until a toothpick inserted into the center comes out clean.
Allow the donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.