01 - Preheat your oven to 350°F (175°C). In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. In a separate large bowl, cream together the softened unsalted butter and granulated sugar until light and fluffy. Beat in the large egg and vanilla extract until well combined. Gradually incorporate the dry ingredients into the wet ingredients until just combined, being careful not to overmix.
02 - Using a 2-tablespoon scoop, portion the dough and roll into balls. Place the dough balls onto prepared baking sheets, flattening them slightly. Bake for 10-12 minutes, or until the edges are lightly golden. Allow the cookies to cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
03 - In a small bowl, whisk together the powdered sugar with 2 tablespoons of milk. Add more milk, a teaspoon at a time, until you achieve a thick, pourable consistency that will drip and create a melted appearance. Spoon the glaze onto the cooled sugar cookies, allowing it to spread and drip down the sides naturally. For the marshmallow hats, gently warm the marshmallows in the microwave for about 10 seconds, just until they begin to slump slightly. Carefully place one warm marshmallow onto the top of each iced cookie. Immediately use mini chocolate chips or candy pearls to create eyes and buttons on the marshmallows and the cookie.