Prepare the Streusel: In a small bowl, combine the melted butter, flour, and sugar. Mix until the mixture resembles coarse crumbs. Set aside.
Make the Muffin Batter: In a large bowl, whisk together the egg, sugar, oil, milk, and vanilla extract until well combined. In a separate bowl, sift together the flour and baking powder. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix. Gently fold in the blueberries.
Assemble the Muffins: Preheat your air fryer to 180°C (350°F). Line muffin cups with paper liners or grease them lightly. Spoon the muffin batter into the cups, filling them about 2/3 full. Sprinkle the prepared streusel topping evenly over each muffin.
Bake: Place the muffin tray in the preheated air fryer. Bake for 14 minutes, or until a toothpick inserted into the center comes out clean and the tops are golden brown.
Cool and Serve: Allow the muffins to cool in the tray for a few minutes before transferring them to a wire rack to cool completely. Enjoy your delicious blueberry streusel muffins with a cup of tea or coffee!