For the crunch topping, place all the ingredients in a bowl and using your fingertips, rub the flour and sugar into the butter until rough crumbs form – avoid over mixing. Set aside.
Preheat the oven to 180˚C/360˚F. Spray two donut pans with cooking spray or wipe with melted butter.
Mix together the eggs, buttermilk, vanilla and melted butter in a mixing jug. In a mixing bowl, stir the flour, baking powder and sugar before making a well and pouring in the wet ingredients. Mix till you have a smooth donut batter.
Pipe or spoon the batter into the prepared tins and sprinkle over the crumble topping. Bake in the oven for approximately 15-20 minutes or until a toothpick inserted comes out clean.
In a small bowl combine the sugar and cinnamon. Once cool enough to handle, turn the donuts out onto a baking sheet. One donut at a time, brush all over with the melted butter and sprinkle generously with the sugar and cinnamon mixture until completely coated. Tap off any excess and devour!