whisk ricotta, stevia and erythritol until very smooth, then incorporate egg yolks one at a time.
add lemon zest and juice and butter and whisk until well blended.
add sifted baking powder and flours.
fold in egg whites.
pour into a buttered silicone mould (20cm x 20cm) and flatten with a spoon or spatula.
sprinkle nuts (optional) on top and bake for approx 25 mins.
allow to cool in the mould, cut into slices and serve.
Notes
Metric kitchen scales are an inexpensive yet invaluable gadget to enable accurate measurement of ingredients. Store them upright in a cupboard or over your worktop and they'll only take up a tiny bit of space. Click HERE for the ones I use.