1. Make the cake layers, following the instructions for the whipped cream cake.
2. While the cake is cooling, make the frosting in the top of a double boiler over simmering water, whisk the sugar and egg whites for 3 minutes. The mixture will look like shiny marshmallow cream. Remove from the heat.
3. Using a handheld mixer on medium speed, beat until the meringue has cooled, about five minutes.
4. Add the butter pats, a third at a time, beating until smooth.
5. Beat the buttercream on medium-high speed for 6 to 10 minutes, until it is thick and very smooth. If the buttercream curdles or separates, just keep beating. It will come together.
6. Lower the speed to medium and gradually beat in the lemon juice, then the vanilla extract and honey. You should have a smooth buttercream. Cover tightly with plastic and set aside.