In a medium mixing bowl, add 145 g all-purpose flour, 50 g graham crumbs, 1 teaspoon baking powder, ¼ teaspoon baking soda, and ½ teaspoon fine sea salt and whisk until combined.
Add 165 g granulated sugar, 150 g mashed ripe banana, 65 g sourdough discard, 45 g whole milk, 30 g unsalted butter, 15 g neutral oil, 40 g sour cream, 2 large eggs, and 1½ teaspoons vanilla extract to a separate bowl. Use a stick blender to blend until smooth.
Pour the dry ingredients over the wet ingredients and mix until just combined. Add 100 g milk chocolate chips or chunks and fold in until evenly distributed.
You may want to rest the batter in the fridge for at least 30 minutes and up to overnight. This is optional, but helps the batter hydrate fully and give the muffins beautiful high tops!
Preheat the oven to 425°F (220°C) and line a standard 12-cup muffin tin with paper liners.
Fill each muffin cup about ⅔ full with batter. Press a frozen marshmallow gently into the centre of each one until just the top is showing. Don’t bury it all the way, because the batter will puff around it as it bakes.
Sprinkle the tops with broken graham cracker bits (and some extra chocolate chips if you're feeling fancy!)
Bake for 6 minutes, then reduce the oven temperature to 350˚F (175˚F) without opening the oven. Bake for 15–20 minutes more or until the tops are golden and a toothpick inserted into the muffin (avoiding the marshmallow) comes out clean or with just a few crumbs attached.
Let the muffins cool in the tin for 5-10 minutes before transferring to a wire rack to cool completely. The marshmallow filling will be hot and melty at first, so avoid eating them for another 10-15 minutes (or be warned that the middle could be really hot!)
