Place your cake in the center of the cake board which has been lined with 6" strips of plastic wrap or waxed paper or parchment paper around the edges. This is to keep the cake board clean while you are frosting the cake. For the sake of recycling, I re-use the plastic wrap which I used to double-wrap and freeze the cake layers. Only place the plastic around the edges so that they are easy to pull out from under the cake when the cake is completed.
Place some buttercream icing in a large piping bag fitted with a large (3/4" opening) round or star tip and pipe a border of icing around the perimeter of the cake layer.
Spread the lemon curd inside the buttercream walls which you have created, but no higher.
Place the second cake layer on top. Cover with plastic wrap and refrigerate until buttercream is firm, about 20 minutes. Then frost the cake as you normally would, trying not to press down on the top of the cake too much, which would force some of the lemon curd out of the middle. Refrigerate the completed cake until one hour before serving to let the buttercream soften a bit. Refrigerate any leftovers (because of the lemon curd filling).
For a Jam-Filled Cake:
The process is essentially the same, except that I spread a very thin layer of buttercream on the bottom cake layer to prevent any of the jam from seeping into the cake. The layer of jam is not intended to be as thick as the lemon curd, so my walls are smaller. I used a smaller round tip with about a 3/8" opening to create the walls here.
Spread the jam on the inside of buttercream wall, place the top layer and continue icing as you would any layer cake. The cake filled with jam can stay at room temperature for up to 2 days.