Preheat the oven to 180 C / Gas 4. Grease a 20cm x 12.5cm loaf tin and line with baking parchment.
Sift the protein powder, flour, bicarbonate of soda and salt into a large mixing bowl.
In a separate bowl, cream the butter and stevia together until light and fluffy.
Add the eggs, mashed bananas, buttermilk and vanilla extract to the butter and stevia mixture and mix well. Fold in the flour mixture. Transfer to the prepared loaf tin.
Bake in the preheated oven for 60 minutes or until a skewer inserted into the centre comes out clean and the loaf is well risen and golden brown. Remove from the oven and cool in the tin for a few minutes, then turn out onto a wire rack to cool completely before serving.