Moist, tender cake donuts topped with mounds of crunchy, brown sugar-cinnamon crumb topping, drizzled with a sweet vanilla glaze.
Preheat oven to 325 degrees. Lightly coat a donut pan with non-stick baking spray and set aside.
In a medium bowl, whisk together sugars, cinnamon, and salt. Pour the butter over top and using a wooden spoon, mix until incorporated. Then add the cake flour and mix until mixture resembles a thick and cohesive dough. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, mix flour, sugar, baking soda, and salt on low speed to combine. With mixer at low speed, add butter one piece at a time; continue beating until mixture resembles moist crumbs, with no visible butter chunks remaining, 1 to 2 minutes.
Add the egg, egg yolk, vanilla extract and buttermilk and mix until all has been incorporated and batter is light and fluffy.
Using a piping bag or spoon, divide up the batter into prepared donut pan. Then, sprinkle a generous amount of the streusel on top, using your fingers to shape the dough into a crumbly mixture. Use a flat palm and gently press the crumb topping into the batter.
Bake for 20 minutes or until the crumbs are golden brown and a toothpick inserted into the middle comes out clean.
Let donuts cool for 5 minutes before transferring them to a wire rack to cool completely.
In a small bowl, mix together the powdered sugar and milk until there are no lumps. Add vanilla extract and stir until smooth. Drizzle the glaze over your cooled donuts. Allow the glaze time to set, then repeat.
Store in an airtight container for up to 2 days.