Preheat oven to 350°F and grease two donut pans.
In a bowl, mix flour, brown sugar, baking powder, salt, and cinnamon.
In a separate bowl, whisk together melted butter, milk, eggs, and vanilla extract.
Add the wet ingredients to the dry and mix until just combined.
Spoon batter into donut pans, filling each cavity about 3/4 full.
In a small bowl, combine crumb topping ingredients and sprinkle generously over each donut.
Bake for 10–12 minutes or until donuts are golden and spring back when lightly touched.
Let cool in pan for 5 minutes, then transfer to a wire rack to cool completely.