Pre-heat the oven to 180C fan-forced and grease and line a large loaf tin or 20x20cm square tin with baking paper so it extends over the sides. In the bowl of an electric mixer, beat the butter and sugar together until light and creamy. Add the egg, yoghurt and vanilla and beat them through. Add the flour, baking powder and salt and mix until well combined.
Spread the mixture into the tin and smooth the top. Bake for 18-20 minutes or until a cake tester inserted in the middle comes out clean and the edges turn golden. Leave to cool completely in the tin.
To make the icing – empty the contents of the packed into the bowl of an electric mixer and add the peanut butter and cocoa. Mix on low and then slowly add milk until it reaches the right consistency. Continue to beat until light and fluffy. Spread the icing on the cooled biscuit and cut into bars.