In a small mixing bowl, combine flour, brown sugar, cinnamon and sugar. Add in the melted butter and mix until well coated and crumbly. Set aside.
Batter:
Preheat the oven to 350°. In a medium-sized mixing bowl, combine flour, cake flour, baking powder, baking soda, cinnamon and salt. Set aside. In a separate large mixing bowl, combine brown sugar, sugar and butter. Whisk by hand until fully combined and smooth. Add in egg and mix to incorporate, then add in the sour cream and mix to combine. Add in milk and vanilla and mix until combined.
Add the flour mixture into the wet ingredients and mix until just incorporated and no dry streaks remain. Spoon the batter into a piping bag. You can use a ziplock bag with the end cut off also. Grease a 6-count donut pan very well with baking spray, then cut the tip of the piping bag and pipe the batter into the donut wells, filling each about halfway full. Sprinkle about 1 Tbsp of crumb mixture onto the top of each donut.
Place the donut pan into the preheated oven on the center rack and bake for 10-12 minutes. You will know the donuts are baked when you gently press on one and it immediately springs back. Once baked, remove from the oven and place onto a wire cooling rack to cool down. While the donuts are still warm, carefully lift the donuts out of the pan so as not to disturb the topping. You can use a smooth-tipped knife to guide the donuts out. Let them cool completely on the rack. Regrease and refill the donut pan with the remaining batter and repeat the baking process.
Icing:
In a small mixing bowl, combine powdered sugar, milk and vanilla. Whisk until smooth. Once the donuts are completely cool, drizzle the icing over top.