Preheat the oven to 325 and lightly grease a donut pan.
Mix together the flour, sugar, baking powder, and cinnamon in a large bowl.
In a small bowl, combine the buttermilk, egg, and butter, whisking to combine.
Add the wet to the dry ingredients and mix well with a wooden spoon. The batter will be thick.
Use two spoons to drop spoonfuls of batter into the donut pan (one to scoop and one to scrape the batter off and into the pan). You should have just enough batter for 6 donuts.
Bake for 10 minutes or until golden and cooked through.
To make the glaze:
While the donuts are baking, whisk together the powdered sugar, vanilla, and milk or cream until smooth. Add more liquid or powdered sugar as needed to get the consistency you prefer.
When the donuts are finished baking and cool enough to handle, dip them in the glaze. Add sprinkles if they make you happy.