Preheat your oven to 350℉. Line your muffin tin with cups.
Make the swirl by mixing the brown sugar, flour, and melted butter together and set aside.
In a bowl, whisk together the flour, baking powder, and baking soda.
In another bowl, mix the melted butter, maple syrup, vanilla, egg, yogurt, and milk. Add this to the flour mixture and mix until almost combined (a few lumps are fine.)
Fold in the chocolate chips.
Spread the swirl mixture over the top of the batter without mixing it. Fold the batter 2-3 times and stop. Over-mixing ruins the swirl.
Divide the batter into your muffin cups until they're about ¾ full.
Bake for 20-22 minutes. Remove, let cool, and enjoy!