Preheat the oven to 170°C (338°F).
Sift the cake flour and baking powder together.
Grate the lemon zest and squeeze the lemon juice.
Leave unsalted butter at room temperature to soften, then mix well with stevia.
Add eggs one at a time, whipping each in.
Add the sifted cake flour and baking powder, lemon zest, and lemon juice to the mixture and blend together.
Pour the batter into a pound cake pan and smooth the surface.
Bake at 170°C (338°F) for 35–40 minutes.
Cool and remove from the pan.