Combine milk and Honey Bunches of Oats® with Cinnamon Bunches cereal in a cereal bowl. Allow to sit for 30 minutes. Strain out milk, measuring 3/4 cup for the doughnuts and reserving the rest for the glaze. (Discard cereal.)
Preheat oven to 425 degrees F. Spray doughnut pan with nonstick cooking spray.
In a large mixing bowl, sift together cake flour, sugar, baking powder, salt and cinnamon. Add reserved cereal milk, eggs, butter and vanilla. Beat until just combined. Spoon doughnut batter in a gallon-size plastic zip top bag. Cut off a 1/2 inch piece from one corner. Pipe into each doughnut cup.
Bake 7-9 minutes or until the top of the doughnuts spring back when touched. Let cool in pan 4-5 minutes before removing.