In a large bowl, whisk together flour, sugar, baking powder, salt and cinnamon. Make a well in the center.
In a large measuring glass, whisk together milk, egg, butter and vanilla extract. Pour into the center of the dry ingredients then mix with a large spoon. Stir until dough is combined but do not overmix. The dough will be sticky! Cover with plastic wrap and set the bowl in the fridge for 1 hour.
After and hour, remove dough and place on a well floured surface. Roll dough out to be about 1/2-3/4 inch thick. Using a doughnut cutter, biscuit cutter or… something round, cut circles into the dough. To cut out the center, I used the wide bottom of a frosting tip, but you can also use a bottle cap or something of the sort.
Heat oil in a large saucepan over medium heat so it’s about 2-3 inches deep. Once the oil hits 360-365 degrees, fry the doughtnuts 2-3 at a time, for about 2 minutes per side, or until puffed and golden brown. Make sure you use a candy thermometer as this is the only way to determine the temperature! I have burned many doughnuts in my life. It is not pretty. Remove doughnuts with a slotted spoon and let cool on a paper towel. Repeat with remaining.
While doughnuts are cooling, heat a small saucepan over medium heat and add butter, vanilla, corn syrup and milk. Whisk to combine and once butter is melt, reduce heat to low and add chopped chocolate. Whisk ask to combine, then once smooth, remove from heat and stir in powdered sugar. Stir continuously until it is all dissolve and no lumps remain. Dunk each doughnut into the glaze, then cover with sprinkles. Let set for about 30 minutes.