Preheat oven to 350°F (175°C) and grease a 6-cavity donut pan.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate bowl, combine the Greek yogurt, egg, applesauce, and vanilla extract until smooth.
Fold the wet ingredients into the dry ingredients until just combined; do not overmix.
Transfer the batter to a piping bag and fill each donut cavity until three-quarters full.
Bake for 8 to 10 minutes until a toothpick inserted comes out clean.
Allow donuts to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Melt the dark chocolate chips and coconut oil together in a microwave-safe bowl in 20-second intervals until smooth.
Dip the top of each cooled donut into the chocolate glaze and set aside until the chocolate hardens.