To make the cinnamon sugar, melt butter in a small saucepan over medium heat. Stir in brown sugar, cinnamon and milk. Bring to a boil; remove from heat.
Preheat oven to 325 degrees F. Coat a donut pan with nonstick spray. Pour 1 teaspoon cinnamon mixture into each cavity, spreading it out in an even layer.
In a large bowl, combine flour, sugar, cornstarch, baking powder, cinnamon, nutmeg and salt.
In a large glass measuring cup or another bowl, whisk together Greek yogurt, butter, milk, vinegar and egg.
Pour mixture over dry ingredients and stir using a rubber spatula just until moist.
Scoop the batter evenly into donut pan. Place into oven and bake for 7-8 minutes, or until donuts are slightly browned and spring back when touched.
In the bowl of an electric mixer fitted with the paddle attachment, beat confectioners’ sugar, cream cheese, milk and vanilla bean seeds on medium-high until smooth, about 1-2 minutes. If the glaze is too thick, add more milk as needed.
When the donuts are done, cool for 10 minutes and drizzle with glaze. Allow glaze to set before serving.