Preheat the oven to 180C/350F degrees
Grease and line a rectangle baking tin with parchment paper.
Beat the egg whites until stiff peaks form.
In another bowl, beat the egg yolks and erythritol together.
Add the peanut flour, cocoa powder, baking powder and salt.
Gently fold in the egg whites and combine thoroughly.
If the mixture is too thick, add the water and mix gently.
Pour into the baking tin and smooth evenly.
Bake for 15 minutes until firm and golden.
Remove from the oven and place on another piece of parchment paper.
Using the parchment paper on the top (will be the one used for baking), gently roll up the cake. Watch your hands and don’t burn yourself! Best to let it cool slightly before rolling. This is to keep the sponge in the correct shape before adding the filling.
Allow to cool.
Gently unroll the cake and remove the top layer of parchment paper.
Remove the peanut butter ice cream from the freezer.
Gently roll the cake around the ice cream cylinder until the two ends meet (it might overlap, depending upon how large it is).
Serve immediately (you might want the ice cream to soften before slicing).