Preheat the oven to 350°. Grease a donut pan and set aside.
In a medium-sized bowl, mix together flour, baking powder, baking soda, cinnamon, and granulated sugar. Mix until combined, then set aside.
In a large bowl, whisk the wet ingredients. Whisk the milk, yogurt, and egg until they are combined. Next, add the melted butter and vanilla continuing to whisk until the items are fully combined. Whisk until smooth.
Slowly add in the dry ingredients to the wet ingredients using a spatula or wooden spoon. You don't want to over mix the batter so it will be pretty thick. Once everything is combined, fold in the sprinkles.
Add the mixture into the prepared donut pan. Fill them about ¾ of the way full. Don't worry if it looks like there isn't enough batter. The batter will fill out significantly in the oven. You can use a spoon to add the batter to the pan but it will be a little messy. It will be easier to transfer the batter to a Ziploc bag and pipe the batter into the pan.
Bake the donuts for about 8-10 minutes. The edges will be golden brown when they are done.
While the donuts are in the oven, make the glaze. In a medium-sized saucepan combine the milk, confectioners sugar, and vanilla. Whisk until the mixture has a smooth consistency. Keep the mixture on low heat. If adding chocolate to the glaze, divide glaze in half and stir in melted chocolate to the mixture.
When the donuts are cool, dunk the donuts into the warm glaze. Make sure the glaze is still warm or the glaze won't stick as well. Transfer the glaze to a wire rack to cool. Make sure you put down a paper towel or sheet pan to catch the falling glaze. It will get a little messy.
Add additional sprinkles if desired and enjoy!
These donuts are best enjoyed fresh. They go stale pretty quickly and don't seem to last beyond a day or so.