Bake about 16 minutes, or till a toothpick inserted near center returns with just a few crumbs. Cool in pan 5 minutes, then turn cupcakes out onto wire rack to completely cool.PEANUT-BUTTER FILLING In small bowl, combine all ingredients. Core cooled cupcakes with a Cuisipro or other hollowing device, and fill cavity with peanut butter filling. Eat the cupcake pieces you cored; sandwich two with some peanut butter filling and marshmallow frosting.MARSHMALLOW FROSTING In a small saucepan fitted with a candy thermometer, bring water, corn syrup and 1/3 cup plus 1 Tbsp. sugar to a rolling boil on high heat, brushing down sides of pan with a wet pastry brush. Do not stir mixture, as it will cause it to crystallize. Boil until mixture reaches 242F, or soft-ball stage.In the meantime, a few minutes after the sugar mixture comes to a boil, in a completely clean, dry mixing bowl. Using regular beaters on medium-high speed whip egg whites, vanilla powder or extract, salt and cream of tartar until creamy and foamy, about 2 minutes. With mixer still on, sprinkle the remaining 1 Tbsp. of sugar over the whites and continue beating until whites hold very soft peaks, another 2 minutes. When syrup has reached 242F, reduce mixer speed to low. Find the sweet spot (between the sides of the bowl and the beaters) and very, very slowly and carefully drizzle in the hot syrup. After all syrup has been added, turn mixer to high and beat till thick, fluffy and just barely warm, about 7 minutes. Fit a 16" piping bag with a 1M Wilton tip; spoon frosting into bag and pipe onto cupcakes making high mounds of frosting.GARNISHES