1) Preheat the oven to 400°F. Lightly grease two baking sheets (three, if you have them), or line them with parchment.
2) Beat the sugar and butter together until creamy.
3) Beat in the baking powder, salt, and extracts.
4) Add the eggs one at a time, beating well and scraping the bowl after each.
5) Add the flour and milk alternately, beginning with about 1 cup of the flour and 1/3 cup of the milk. Beat well after each addition. The final batter should be fairly thick and fluffy.
6) Scoop the batter onto the prepared baking sheets in heaping tablespoonfuls; a tablespoon cookie scoop is a useful tool here. Leave about 2" between each scoop.
7) Bake the cakes for 7 to 8 minutes, or until they're springy to the touch and feel set. Remove them from the oven, and let them cool right on the pan; or transfer them to a rack to cool.
8) To make the filling: Place the filling ingredients, except the extracts, in a microwave-safe bowl or a saucepan, stirring to combine.
9) Heat gently (in the microwave, or over a burner), stirring frequently, until the marshmallows melt completely.
10) Remove from the heat, and stir in the extracts.
11) Pour the icing into the bowl of your stand mixer or another mixing bowl, and refrigerate overnight, or until thoroughly chilled.
12) Remove the icing from the refrigerator. Beat it at high speed, using your stand mixer or an electric hand mixer; it's impossible to beat thoroughly enough by hand, so please use a machine. Scrape the sides and bottom of the bowl to make sure you're incorporating all of the chilled icing.
13) Beat only until the icing is smooth and glossy; this will probably take less than a minute. The icing will thin out and become almost sauce-like. Once it's smooth, set the icing aside to rest at room temperature for 30 to 45 minutes; it'll stiffen as it sits, bringing it to spreadable consistency.
14) Spread the flat side of half the completely cooled cakes with the filling; very slightly heaping a tablespoon cookie scoop (about 5 measuring teaspoons) is the right amount.
15) Top with the remaining cakes, flat side towards the filling.
16) Serve whoopie pies immediately; or cover lightly with plastic wrap, and enjoy within a couple of days. For longer storage, wrap individually, and freeze.
Yield: about 18 medium (2 1/2") whoopie pies.