Preheat oven to 325 degrees.

Spray non-stick donut pan (6 donut size) with cooking spray and set aside.

In a large bowl, whisk together the flour, baking powder, salt and sugar.

In a smaller bowl, whisk together the yogurt, milk, egg, vanilla and melted butter.

Make a well in the dry ingredient bowl and pour the wet ingredients into the center.

Using a spatula, stir gently until no flour streaks remain. It is a thick batter.

Scoop two tablespoons of batter into each donut well and spread evenly.

Bake on center rack of oven for 12 - 13 minutes, until donuts feel springy when touched.

Allow to cool in pans for 10 minutes, then remove to cooling rack.

Meanwhile, brown the remaining 3 tablespoons of unsalted butter in a small skillet over medium heat, swirling the pan often until the bits in the bottom are browning and smelling nutty. Remove from heat and put into a shallow bowl.

In a separate shallow bowl, whisk together the sugars and cinnamon. Brush the tops of each donut with the browned butter until well coated and then dip the tops to coat generously with the sugar and cinnamon mixture. Store in a air-tight container until ready to serve. Yields 6 donuts.