Preheat oven to 400°F. In a large bowl, combine flour, baking powder, sugar and salt. Add greek yogurt, egg, milk, butter and vanilla extract and stir until just combined. Stir in sprinkles at very end.
Place batter in ziploc bag and squeeze out air. Cut corner of bag. Spray donut pan with non-stick spray. Pipe the batter evenly in the donut pan. It should make 6. Bake for 8-9 minutes or until stick comes clean. These are a sponge-like consistency so don't expect it to be really browned on top. Remove from oven and remove from pan and cool on a cooling rack.
Vanilla Glaze
For the glaze, add the sugar to a shallow bowl. Slowly stir in milk and vanilla extract to make smooth, pourable glaze. Dip each donut in the glaze and top with sprinkles. Allow glaze to set for 30 minutes before serving.
Notes
Even though they say non-stick, spray your donut pans with non-stick cooking spray to make sure the donuts come out.
When measuring the flour, lightly spoon it into a measuring cup and then level it off with the back of a knife to get the right amount.
Wait until the donut batter is completely mixed before folding in the sprinkles as it can bleed the colors.
You can spoon the batter into the donut pans, but the easiest way to get the batter in the pan is by using a pastry bag or Ziploc bag with the corner cut off.
Let the donuts cool completely before you dip them in the glaze.
Leftovers will store well in at room temperature for 1-2 days in an air-tight container.