Preheat the oven to 180°C (356°F).
Soften the butter.
Mix the stevia, butter, and salt well.
Add the egg and mix.
Sift together cake flour, cocoa powder, and baking powder, then add to the mixture and combine.
Shape the dough into bite-sized balls.
Place the cookie dough onto a baking tray.
Bake in the oven at 180°C (356°F) for 12-15 minutes.
Let cool before serving.