Preheat your oven to 350F/175C and grease your doughnut pan. In a large bowl, whisk together your dry ingredients (flour, sugar, cinnamon, baking powder and soda) until evenly mixed. In a small bowl, whisk together your wet ingredients (butter, egg, milk, and vanilla extract). Whisk the wet ingredients into the dry, and continue whisking until your batter is lump-free. Pour or pipe the doughnut batter into your greased doughnut pan and bake for 9-10 minutes, or until a toothpick inserted in your doughnuts comes out clean.
Turn the baked doughnuts out onto a metal cooling rack once the pan is cool enough to handle.
In a small saucepan, combine your powdered sugar, milk, and vanilla extract over low-medium heat, whisking occasionally until a skin starts to form (when you stir you'll see the top start to crack). Dip your cooled doughnuts into the glaze (stirring before each dip) and garnish immediately with crushed pieces of Cinnamon Toast Crunch cereal.