Preheat oven to 350 degrees and spray a donut pan with non-stick spray, set aside. 

In a medium sized bowl combine flour, baking powder, ½ tsp cinnamon, and salt. Set aside.

In a large bowl combine egg, sugar, milk, and vanilla. Whisk together well.

Add flour mixture and stir until just combined.

Pour your batter into your prepared donut pan and set aside.

In a small bowl combine all ingredients for the cinnamon topping. This should be runny! (If it's not that means your butter cooled too much- just pop it in the microwave for a few seconds) Then, drizzle it evenly over the top of the donuts (it won’t cover them all- that’s okay!)

Now, take a knife and being to “swirl” in your cinnamon sugar mixture to the donut batter. Be sure that none of the cinnamon mixture “pools” at the edges (this will cause it to spill out) fold it into the batter. 

Bake for 10 minutes or until golden brown and a toothpick when inserted comes out clean. Allow donuts to cool fully before adding frosting. 

In a small bowl melt your butter (it can be still warm for this!) Then add in the powdered sugar and milk, stirring until you have a uniform frosting. (if your frosting is clear you’ll need to add more powdered sugar- you want to see it!! If it's tooo thick and won't drizzle add just a few drops of milk at a time.. really.. drops!)

Drizzle your frosting over the donuts immediately (if you wait too long your butter will cool and harden)

Enjoy!