Prepare Swiss meringue buttercream as shown in this recipe and video
Add lemon extract to the buttercream - combine well
Once cooled - cut off any dome on the cake to level them
Spread buttercream and sandwich the two layers
Crumb coats or spread a little buttercream around the cake. (this helps seal the crumbs)
Chill the cake if necessary for 10 to 15 minutes so the layers won't move.
Add yellow gel food color to the remaining buttercream
Put buttercream in a piping bag with a 104 or rose piping tip nozzle
Pipe around the cake in a zig-zag upwards motion holding the piping bag vertical against the cake.