Preheat your oven to 350°F (175°C).
Grease your donut pan with non-stick spray or a light coating of butter.
In a large mixing bowl, whisk together the all-purpose flour, dark brown sugar, salt, baking powder, and ground cinnamon until well combined.
In a separate bowl, whisk together the milk, melted unsalted butter, and the large egg until smooth.
Pour the wet ingredients into the dry ingredients, and gently mix until just combined.
Spoon the batter into the prepared donut pan, filling each mold about 2/3 full.
Bake in the preheated oven for 10-12 minutes or until golden brown.
While the donuts are baking, prepare the glaze by mixing confectioners' sugar, milk, and cinnamon until smooth.
Let the donuts cool in the pan for about 5 minutes before transferring to a cooling rack. Dip the tops into the glaze.
Serve warm or at room temperature.
