Special equipment: 12 paper cupcake liners, a pastry bag fitted with a number 866, 867 or 868 star tip, a 805 or 806 straight piping tip and a 2-inch circle cookie cutter
For the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with paper liners.
Whisk the flour, baking powder and salt in a medium bowl. Beat the butter and granulated sugar in a stand mixer fitted with the paddle attachment on medium-high speed until fluffy, about 4 minutes. Beat in the eggs, one at a time, scraping down the bowl as needed. Beat in the vanilla. Reduce the mixer speed to medium-low; beat in half of the flour mixture, then all of the milk, then the remaining flour mixture until just combined.
Transfer one-third of the batter to separate medium bowl. Add the pink or blue food coloring and stir until uniform in color. Using an ice cream scoop, add scoops of colored batter back to the bowl with the plain batter in an evenly spaced circle. Run a butter knife or skewer through the batters to create a swirled and marbled look.
Divide the batter evenly among the prepared muffin cups using the same ice cream scoop, filling each three-quarters full. Bake, rotating the pan halfway through, until a toothpick inserted into the center of a cupcake comes out clean, 18 to 20 minutes. Transfer the pan to a rack and let cool for 5 minutes, then remove the cupcakes to the rack to cool completely.
For the vanilla American buttercream: Combine the butter, confectioners' sugar and salt in the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl if using a handheld electric mixer) and beat on low speed until mostly incorporated. Add the vanilla, increase the speed to medium-high and beat until smooth. Adjust the consistency with milk as desired. Use immediately or refrigerate in an airtight container up to 3 days. (If refrigerated, allow to come to room temperature and then beat on medium-high speed until smooth before using.)
For the assembly: Use yellow food coloring to tint the buttercream pale yellow. Fill a pastry bag fitted with a large star tip (such as number 866, 867 or 868) with the buttercream. Pipe buttercream in a swirl over the top of each cupcake to cover it completely.
Sprinkle a work surface with cornstarch and roll out the skin-tone fondant to roughly 1/8 inch thick. Cut out 4 circles with a 2-inch circle cutter. In the middle of each circle, create eyes by making 2 half-moon indentions with the small end of a straight piping tip (such as number 805 or 806). Roll 4 smalls balls of fondant to make noses, brushing them with water to secure them to the face. Set the faces aside.
Roll out the pink and/or blue fondant to about 1/8 inch thick. Use the 2-inch circle cutter to cut out 2 circles. Use a knife to cut the circles in half to form hats. Cut a small strip from the flat end of each semicircle and stack it on the bottom of each hat to make a border, securing with water. Brush additional water onto the back of the hats and secure them to the faces.
Cut 4 small circles of pink fondant using the small end of the straight piping tip. Brush with water and attach to the faces as the base of a pacifier. Roll 8 small balls of pink fondant. Use 4 to attach to the hats as pompoms and 4 to attach to the small circles to complete the pacifiers. Lay the fondant faces on top of cupcakes.