Preheat oven to 375° F. Spray a nonstick muffin tin with cooking spray.
Put the butter in a large microwave-safe bowl and melt in the microwave. (Or melt the butter in a saucepan on the stove and transfer to a large bowl.)
⅓ cup (75g) unsalted butter
Add the honey, mashed banana, eggs, milk, plain Greek yogurt and vanilla to the bowl. Whisk until well combined.
½ cup (168g) honey, 1 ¼ cups (280g) mashed ripe banana, 2 large eggs, ¼ cup (60mL) milk, ¼ cup (57g) plain Greek yogurt, 1 teaspoon vanilla extract
Add the baking powder, baking soda, cinnamon and salt to the bowl. Whisk until very well combined, making sure there are no lumps of baking powder or baking soda left in the batter.
2 teaspoons baking powder, ½ teaspoon baking soda, 1 ½ teaspoons ground cinnamon, ½ teaspoon salt
Add the white whole wheat flour to the bowl and use a rubber spatula to mix in the flour until just combined. Then fold in the chopped apple. Be careful not to over-mix the batter, or your muffins could turn out tough or dense.
2 cups (240g) white whole wheat flour, 1 ½ cups (160g) diced fresh apple
Divide the batter evenly among 12 muffin cups, filling each almost to the top. Bake muffins for 16 to 20 minutes, until a tester inserted into the center of a muffin comes out clean. Let muffins cool in the pan for 5 minutes and then transfer to a wire rack to finish cooling.