Set your oven to 375°F and line or grease a 12-cup muffin tin.
Mix dry ingredients. In a big bowl, whisk together flour, salt, sugar, and baking powder.
Mix wet ingredients. In a separate bowl, whisk milk, egg, oil, and vanilla.
Add dry to wet and stir until just combined—don’t overmix!
Add the goodies. Gently fold in mini marshmallows and chopped chocolate.
Fill muffin cups. Divide the batter evenly (about ¾ full). Add a few more marshmallows and chocolate bits on top.
Top with crumble. Mix your graham cracker crumbs, brown sugar, and melted butter. Sprinkle that over each muffin.
Pop them in the oven for 15–20 minutes, until golden and a toothpick comes out clean.
Let them sit in the pan for 5 minutes before moving to a rack. Serve warm.