Preheat your oven to 350°F (175°C) and prepare a 6-mold donut pan with non-stick cooking spray.

In a medium bowl, mix 1/4 cup granulated sugar, 3.5 tablespoons all-purpose flour, and 1/4 teaspoon cinnamon. Incorporate 2 tablespoons cold unsalted butter until crumbly and chill in the freezer.

In a large bowl, combine 1 cup all-purpose flour, 1/2 cup packed light brown sugar, 1 teaspoon baking powder, and 1/2 teaspoon salt. In another bowl, mix 1/2 cup full-fat sour cream, 2.5 tablespoons melted unsalted butter, and 1 large egg, then fold into dry ingredients.

Spoon or pipe donut batter into the pan, filling each mold three-quarters full. Sprinkle chilled crumb topping over the batter.

Bake for 10-11 minutes until lightly golden brown and springy to touch. Allow to cool in the pan for 10 minutes before transferring to a wire rack.

In a small bowl, whisk together 1/2 cup confectioners' sugar, 1/2 tablespoon whole milk, and 1/2 teaspoon vanilla extract until smooth.

Drizzle the glaze over cooled donuts and serve.