Cupcakes: Preheat oven to 350 degrees. Line two standard muffin tins with paper liners.

In a medium bowl, whisk together flour, baking powder, and salt. In the bowl of a mixer fitted with the paddle attachment, beat together butter and sugar on medium-high speed until light and fluffy, 2 to 3 minutes. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Reduce speed to low and add flour mixture in three batches, alternating with milk, beginning and ending with flour and beating until combined after each addition.

Divide batter evenly among muffin cups, filling each about three-quarters full. Bake, rotating tins halfway through, until a tester inserted into centers comes out clean, about 20 minutes. Let cool in tins 10 minutes, then transfer cupcakes to wire racks and let cool completely. Cupcakes can be stored in airtight containers in freezer up to 1 week. Bring to room temperature before using.

Buttercream: Combine egg whites and sugar in the heatproof bowl of a mixer set over (not in) a pot of simmering water. Whisk constantly until sugar is dissolved and mixture is warm to the touch. Remove from heat and transfer bowl to mixer fitted with the whisk attachment. Whisk on high speed until stiff (but not dry) peaks form and mixture is cool to the touch, about 6 minutes. Whisk in vanilla.

Switch to paddle attachment. With mixer on low, add butter, 2 tablespoons at a time, beating well after each addition. Continue beating until smooth, about 3 minutes. Buttercream can be stored, covered, in refrigerator up to 5 days or in freezer up to 2 months. Before using, bring to room temperature (if frozen, thaw overnight in refrigerator first) and beat on low speed until smooth.

Assembly: Frost each cupcake with about 1/4 cup buttercream, smoothing tops with an offset spatula. Transfer remaining buttercream to a resealable plastic bag. Snip a very small hole in one corner of bag.

Using kitchen shears, cut a 1/4-inch slice off the top of 24 marshmallows. Using your fingers, press down on each to condense and make slightly smaller in size. Pipe a small dot of buttercream on the tops of 24 whole marshmallows. Place a cut marshmallow on top of each to create heads. Pipe another small dot of buttercream on top of each head, then add mint candies to create hats (reserve remaining buttercream for another use). Repeat with remaining marshmallows.

Using a wet toothpick, poke two parallel holes in each head and insert sprinkles to create eyes. Poke another hole below the eyes and insert a sunflower seed to create nose. Repeat with remaining snowmen. Top frosted cupcakes with snowmen. Decorated cupcakes can be stored in airtight containers at room temperature up to 1 day.