Preheat your oven to 350°F (175°C). Lightly grease two donut pans with non-stick cooking spray.

In a medium mixing bowl, whisk together the all-purpose flour, cake flour, cocoa powder, baking powder, and salt. In a separate small bowl, whisk together the sour cream and milk. In a separate large mixing bowl, whisk together the eggs and sugar, followed by the milk mixture, melted butter, and vanilla extract until well combined.

Pour the dry ingredients into the bowl with the wet ingredients and whisk until just combined. Be careful not to overmix.

Spoon the batter into a piping bag or a resealable plastic bag with the corner snipped off. Pipe the batter into the prepared donut pan, filling each cavity about 2/3 full.

Bake the donuts for 9 to 10 minutes, or until the tops spring back when lightly touched or a toothpick inserted into the center comes out clean. Remove the donuts from the oven and turn out immediately onto a wire rack to cool.

For the glaze, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until smooth and well combined. Add more milk if needed to achieve your desired consistency.

Dip the slightly warm donuts into the glaze, allowing any excess to drip off, and place them back on the wire rack. I like putting the crinkled tops facing up because they resemble fried chocolate cake donuts!