Step 1: Preheat your oven to 350°F (175°C). Lightly grease your donut pan with non-stick spray or butter.
Step 2: In a medium bowl, mix together the cold unsalted butter, granulated sugar, all-purpose flour, and cinnamon until it resembles wet sand. Set aside.
Step 3: In a large bowl, whisk together 1 cup of all-purpose flour, light brown sugar, salt, and baking powder until well combined.
Step 4: In another bowl, combine the full-fat sour cream, melted butter, and egg. Whisk until smooth.
Step 5: Pour the wet mixture into the dry ingredients. Gently fold together until just combined.
Step 6: Spoon the batter into the prepared donut pan, filling each cavity about 2/3 full.
Step 7: Sprinkle the crumb topping over each donut before baking.
Step 8: Bake for about 12-15 minutes, or until a toothpick comes out clean.
Step 9: Allow the donuts to cool in the pan for about 5 minutes, then transfer to a cooling rack.
Step 10: In a small bowl, whisk together the confectioners' sugar, whole milk, and vanilla extract until smooth.
Step 11: Once cool, dip the tops into the glaze or drizzle it over the donuts.