Preheat oven to 350 degrees F. 180 C.
Prepare pan. Place a parchment paper in bottom and spray or butter bottom and sides.
Zest lemons. Add that to the sweetener. Juice lemons. Set juice aside.
Make a bain marie. Set a stainless steel bowl over a pan of boiling water (not touching). Turn water down to simmer.
Melt white chocolate and butter in bain marie, stirring with wooden spoon. Set aside.
Beat eggs whites into soft peaks. Set aside and clean mixer for egg yolks.
Beat egg yolks with sweetener until creamy, about 4 minutes.
Add white chocolate mixture, lemon zest, lemon juice.
Add dry ingredients.
Fold in the egg whites.
Pour into prepared pan.
Bake for 30 to 40 minutes until golden brown. The cooking time will depend upon the size of your pan. I used a 10 inch pan with removable bottom and it only took 30 minutes. The torta is not high and I prefer it like this. You can use a 9 inch pan.
Check at 25 minutes. If the cake is browning too much, cover with foil.
Serve with whipped cream.