Heat oven to 350°F. Line two large baking sheets with parchment.
Whisk flour, baking powder, baking soda, and salt in a bowl; set aside.
Beat softened butter and sugar until pale and fluffy, about 2 minutes.
Mix in egg and vanilla until smooth and glossy.
Add dry ingredients and buttermilk in turns, beginning and ending with dry. Mix just to combine.
Fold in sprinkles gently so they don’t streak the batter.
Scoop 2-tablespoon mounds onto sheets, spacing 2 inches apart for spreading.
Bake 10–12 minutes until tops spring back and edges look set. Cool completely on racks.
For the filling, beat butter, powdered sugar, and salt until creamy and light.
Beat in marshmallow creme and vanilla. Add cream a splash at a time until spreadable.
Pair cooled cakes by size. Spread or pipe filling on one, top with its mate, and gently press. Roll edges in sprinkles if desired.