Pre-heat oven to 350F (180C), grease and flour a 6 size donut pan.
In a small pot melt butter and let cool.
In a medium bowl whisk together flour, baking powder, baking soda, salt, sugar and cinnamon.
In a medium bowl whisk together egg, milk, yogurt, vanilla and melted butter until smooth.
Mix the wet ingredients into dry ingredients and mix just until combined, do not over mix. It there are a few small lumps that is ok.
Place the batter in a pastry bag and fill almost to the top the donut pan (or use a spoon to fill the pan). Bake approximately 10 minutes.
In a small bowl combine well the sugar and cinnamon, set aside.
Remove from oven, wait approximately 3-5 minutes then flip them over and dip the donuts in the cinnamon sugar topping or let cool then brush with melted butter (approximately 2 tablespoons/28.75 grams) and dip in topping. Enjoy!
For room temperature ingredients, remove from the fridge 45-60 minutes before using.