01 - Lightly grease a microwave-safe 12-ounce mug with cooking spray or butter.
02 - Add flour, sugar, and baking powder to the mug. Whisk gently to combine and break up any lumps.
03 - Stir in milk, melted butter, and vanilla extract until a smooth batter forms.
04 - Sprinkle graham cracker crumbs and mini chocolate chips evenly over the batter. Top with mini marshmallows.
05 - Microwave on high power for 1 minute 20 seconds to 1 minute 40 seconds, until the cake is set and marshmallows are puffed.
06 - Remove from microwave and immediately sprinkle with a pinch of sea salt flakes. Allow to cool slightly before serving.