Set your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease with nonstick spray.
In a large mixing bowl, whisk together the flour, salt, granulated sugar, and baking powder.
½ cup all-purpose flour, ½ teaspoon salt, ½ cup granulated sugar, 2 teaspoons baking powder
In a separate bowl, whisk the milk, egg, vegetable oil, and vanilla extract until well blended.
¾ cup milk, 1 large egg, ½ cup vegetable oil, 1 ½ teaspoons vanilla extract
Pour the wet ingredients into the dry ingredients and stir just until combined—do not overmix. Gently fold in the mini marshmallows and chopped chocolate.
½ cup mini marshmallows, ½ cup chopped semi-sweet chocolate
Divide the batter evenly into the prepared muffin cups, filling each about ¾ full. Top each muffin with a few extra marshmallows and chocolate pieces for gooey goodness.
In a small bowl, mix the crushed graham crackers and brown sugar, then stir in the melted butter until crumbly. Sprinkle the mixture evenly over the tops of the muffins.
½ cup crushed graham crackers, 1 tablespoon light brown sugar, 2 tablespoons melted butter
Bake for 15 to 20 minutes, or until the muffin tops are golden and a toothpick inserted into the center comes out clean. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack.
Serve warm to enjoy the melty chocolate and toasted marshmallow flavor!