In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter and the sugar until light, fluffy and creamy, about 2-3 minutes.
Add the vanilla and the eggs, one at a time, and mix after each addition, just until combined.
In a medium bowl, whisk together the flour, baking powder and salt. Add the flour mixture to the butter mixture and mix on low just until the flour is incorporated.
Remove the bowl from the stand mixer. Stir in the sprinkles.
Use an ice cream scoop to evenly portion the dough into balls and place on baking sheets, leaving two inches between the cookies. Chill in the refrigerator for 30 minutes.
While the cookies are chilling in the fridge, heat the oven to 375F. Line two cookie sheets with parchment paper.
Bake the cookies for 11-13 minutes or until the edges are just beginning to turn a light, golden brown. The centers should be just set. Cool completely.
While the cookies are cooling, prepare the marshmallow buttercream filling. In the bowl of a stand mixer fitted with a paddle attachment, mix the butter and confectioners sugar on low until combined.
Add the salt, vanilla bean paste, and milk, mixing on low until incorporated. Add in the marshmallow cream.
Turn the mixer to medium high and whip the buttercream, scraping the bowl every two minutes or so, for 8 minutes, until super light, fluffy and creamy.
Transfer the buttercream to a piping bag fitted with your choice of piping tip. Pipe a circle of buttercream on half of the cookies, leaving a bit of space (about 1/4 inch) around the edge of the cookie.
Top with remaining cookies, pressing the sandwiches together so that the buttercream is near the edges.
Roll the edges of the sandwich cookies in sprinkles. At this point you can refrigerate the cookies to firm up the buttercream or serve as is.