1. Pre-heat the oven to 180C/350F/gas mark 4. Line a 24 cm square cake tin with non-stick baking paper.
2. Grate the lemon zest with a fine grater and set aside.
3. Mix the polenta, ground almonds and baking powder in a bowl.
4. In another bowl, whisk the butter and xylitol with an electric whisk for a few minutes.
5. Add the eggs one by one, whisking thoroughly each time.
6. Fold in the dry ingredients and vanilla extract.
7. Stir in the raspberries, blueberries and lemon zest.
8. Spoon the mixture into the baking tin and level with the back of a spoon.
9. Bake for 30-35 minutes or until the top looks golden and a skewer inserted in the middle comes out clean.
10. Once cool, cut into squares.
Copyright © Tarja Moles 2012.
Tarja Moles is the author of No Naughties: Sweet treats without sugar, wheat, gluten and yeast. You can find more information on living with dietary restrictions and free recipes on www.nonaughties.com