Preheat your oven to 350°F (175°C). This ensures that your muffins bake evenly from the start.
Line a muffin tin with parchment paper liners or lightly grease it with cooking spray.
In a large mixing bowl, whisk together the melted butter, maple syrup, vanilla extract, and eggs until well combined. Add the Greek yogurt and milk, mixing until smooth.
In another bowl, whisk together the flour, baking powder, baking soda, salt, and dark brown sugar until evenly mixed.
Gradually add the dry ingredients to the wet ingredients, stirring with a spatula until just combined.
Toss the mini and semi-sweet chocolate chips with the tablespoon of flour to coat them. Gently fold the chocolate chips into the batter.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
In a small bowl, mix the melted butter with the apple butter. Brush this mixture on top of the muffin batter.
Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the muffins from the oven and allow them to cool in the tin for about 5 minutes before transferring them to a cooling rack.
