01 - Preheat oven to 350°F (175°C). Lightly grease a nonstick doughnut pan.
02 - In a large bowl, whisk together all-purpose flour, protein powder, granulated sugar, baking powder, ground cinnamon, baking soda, and salt until evenly combined.
03 - In a medium bowl, beat eggs. Add Greek yogurt, applesauce, milk, melted coconut oil or butter, and vanilla extract. Mix until smooth and homogeneous.
04 - Pour wet mixture over the dry ingredients. Using a spatula or spoon, gently stir until just combined. Avoid overmixing.
05 - Carefully fold mixed berries into the batter to distribute evenly.
06 - Spoon batter into prepared doughnut pan, filling each cavity about three-quarters full.
07 - Bake in preheated oven for 12 to 15 minutes, or until a toothpick inserted into a doughnut comes out clean.
08 - Remove pan from oven and allow doughnuts to cool for 5 minutes. Transfer doughnuts to a wire rack to finish cooling.
09 - While doughnuts are slightly warm, use a pastry brush to coat each with melted butter, then sprinkle or dip into cinnamon sugar mixture.
10 - Serve warm or at room temperature. Store any leftovers in an airtight container for up to 2 days.