Preheat oven to 375 degrees.
In a medium bowl, mix flour, baking powder, cinnamon and salt.
In a large bowl, cream softened butter and sugar together and beat in egg.
Add milk and vanilla to large bowl with butter, then add the flour mixture, mixing until batter is thoroughly combined.
Spoon into a greased donut pan.
Bake for 12 minutes and remove from oven.
While donuts are still warm, dip in a mixture of cinnamon and sugar.