Fit piping bag with medium round tip. Fill piping bag with about ½ cup buttercream. Set aside.
Level cooled cake layers as necessary. Place one cake layer on cake board, serving platter, or cake turntable. Brush or lightly drizzle soaking syrup over cake layer. Spread a thin layer of buttercream on top using a small offset spatula.
Using the filled piping bag, pipe a border of buttercream along the perimeter of the cake round. Pipe another inner circle of buttercream to create a double wall barrier.
Add half of cooled apple filling within buttercream barrier. Place second cake layer on top. Repeat soaking cake, spreading thin layer of buttercream, piping border, and adding apple filling.
Place third cake layer on top. Brush any remaining soaking syrup over cake layer. Spread buttercream around the cake to create a "crumb coat" layer to seal in any crumbs. Chill cake in the fridge for 15-20 minutes to set frosting. If desired, generously coat the chilled cake with additional buttercream for a smooth finish.
Fit a piping bag with a medium star tip. Fill bag with any remaining buttercream. Decorate the top of cake with piped star boarder. Garnish with sprinkles as desired. Store cake in the fridge until ready to serve.