• In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
• Add the flour and baking powder and mix until combined. The dough should be fairly solid at this stage (it ought to almost form a ball, albeit a very soft one!). Scoop large tablespoons of the dough and wrap around 1-2 marshmallows (if using very big ones, cut them in half first). Don't leave much marshmallow exposed of they will leak out during baking. Place in fridge to chill for 30 minutes.
• Preheat the oven to 180℃/375℉. Line two-three large baking sheets.
• Place cookies 2 inches apart on baking sheets – they spread! Bake 12-14 minutes, until just golden at the edges. Let cool.

For the Frosting

• Once the cookies are cool, make the frosting. Add the butter to a large mixing bowl. Beat with a whisk on high for a minute to soften/combine everything.
• Add half the icing sugar, and the vanilla and whisk on high. Then add the remaining half and whisk everything on high until fluffy. Add icing sugar/milk as required to meet your desired consistency. Spread over the cooled cookies, and top with a sprinkle of Lucky Charms Cereal!