Preheat your oven to 350°F (175°C). This is crucial for ensuring your tarts bake evenly.
In a mixing bowl, combine the 1/2 cup salted butter and 3/4 cup white sugar. Using an electric mixer or a whisk, cream them together until light and fluffy, about 2-3 minutes.
Beat in the 1 egg and 1 tsp. vanilla extract until fully incorporated.
In a separate bowl, whisk together the 1 cup all-purpose flour and 1 tsp. baking powder. Gradually add this dry mixture to the wet ingredients, mixing until just combined.
Fold in the 1 cup graham cracker crumbs until the mixture resembles a soft dough.
Grease your tart pans or muffin tin lightly. Using a spoon or your hands, distribute the dough evenly into the pans, pressing it firmly to create a crust that goes up the sides.
Bake the crusts in the preheated oven for 10-12 minutes or until they are lightly golden. Remove from the oven and let them cool slightly.
Once the crusts have cooled for a few minutes, place the unwrapped chocolate covered marshmallows into each tart shell, pressing them down gently.
Return the tarts to the oven and bake for an additional 5-7 minutes, or until the marshmallows are soft and slightly melted.
Allow the tarts to cool completely in the pan before gently removing them. Serve warm or at room temperature and enjoy your delicious Smore Cookie Tarts!