Preheat your oven to 350 degrees, and line two large baking sheets with parchment paper. Set aside. In a large mixing bowl, beat together the butter and sugar until light and fluffy. Beat in the vanilla extract and eggs until combined. Add in the flour, salt, and baking powder, and beat until a dough forms. It will be fairly firm, and you want to beat it for several minutes to ensure that all the flour is completely incorporated.
Form your dough into a long log that is about 1 1/2 inches around. Wrap in plastic and chill for 1 hour. Unwrap the chilled dough, and slice the cookies into 1/4 inch thick rounds. Place them on your lined baking sheet, no need to leave much space between each cookie, just a little. Bake cookies for 8-10 minutes, or until they are just starting to become golden on the edges. Don't overbake these ones- you want them to stay soft! Continue with the remaining dough until all the cookies are baked. Cool completely.
In a large bowl, or the bowl of stand mixer (which I recommend using if you have), stir together the egg whites and sugar over a double-boiler. You want to heat this while whisking for about 5-10 minutes, or until the sugar is dissolved and you can't feel any grains when you rub a little between your fingers.
Then, transfer to your stand mixer, or using a hand mixer, use the whisk attachment and beat the mixture for a full 10-15 minutes. The mixture will triple in size, and will become very thick and fluffy. You want to make sure you beat this mixture fully- it should be very thick, and should hold peaks!
Transfer the mixture to a piping bag fitted with a star tip, and generously pipe marshmallow on top of each cookie into a dollop, pulling away at the end to create a little peak on top. Then, freeze the cookies for about 20-30 minutes to help the meringue become even more firm for dipping in chocolate.
In a microwave safe bowl, add about 2/3 of your chopped chocolate. Microwave in 30 second bursts, stirring between each burst, until the chocolate is completely melted. Then, stir in the remaining chopped chocolate until that is also completely melted, which should take a minute or two or stirring.
Then, gently dip each marshmallow topped cookie into the melted chocolate, and sprinkle your sprinkles on top before the chocolate sets. Then your cookies are ready to enjoy! Then can be stored at room temp for 3 days, or you can freeze the cookies for several weeks. Enjoy!