Mix dry ingredients: In a medium bowl, whisk together flour, sugar, baking powder, salt, and cinnamon.
Combine wet ingredients: In a separate bowl, beat eggs lightly, then whisk in milk, melted butter, and vanilla.
Make batter: Pour wet mixture into dry and stir until just combined. It’s okay to leave a few lumps. Cover and let rest for 5–10 minutes.
Prep packs: Lay out foil squares. Spoon about ½ cup of batter into center of each.
Add bananas & toppings: Arrange 4–6 banana slices atop batter. Add a pat of butter, a sprinkle of brown sugar or chocolate chips, and nuts if desired.
Seal: Fold foil edges twice to create a pouch, leaving a small vent for steam.
Grill: Preheat grill to medium (about 350°F). Place packs on indirect heat and cook 12–15 minutes, flipping halfway, until packets feel springy and a toothpick inserted comes out clean.
Rest & serve: Remove from heat, let rest 2–3 minutes, then carefully open and enjoy.