To make the cupcakes:Â Preheat oven to 350 degrees (180 C). Line a muffin pan with cupcake liners; set aside.
In a large measuring cup, whisk together the eggs, milk, and vanilla extract; set aside.
In a large bowl, or the bowl of a standing mixer, combine the flour, sugar, baking powder, and salt. With the mixer on low, add the butter one piece at a time, mixing until the dough comes together and looks sandy.
Add about 1/2 of the egg mixture and mix on low speed until incorporated, then increase to medium-high and beat until light and fluffy. Slowly add the remaining egg mixture, with the mixer on low speed, until incorporated. Scrape the bowl, if necessary, then increase speed to medium-high and beat for about 20 seconds. The batter will just barely look a little curdled.
Put 3 tablespoons of batter in each cupcake liner. Bake for 20-23 minutes, or until a toothpick inserted in the center comes out clean. Cool completely before filling and frosting.
To make the filling:Â In a medium bowl, cream the butter and sugar together. Stir in the vanilla and pinch of salt. Mix in the marshmallow spread and whisk until light and fluffy. Cover bowl with plastic wrap and chill for 30 minutes. If the filling is too soft or runny, add more sugar as needed (I had to use 1 cup of sugar total).
To make the frosting:Â In a large bowl, or the bowl of a standing mixer, beat the butter and marshmallow until light and fluffy, about 5 minutes. Add the sugar, beating until just combined. Stir in 1 tablespoon of cream and the vanilla, then beat on high speed for 4 minutes. If the frosting is too thick, add more cream, up to 1Â tablespoon.