Preheat the oven to 170°C (150°C fan-forced) and lightly spray a 20 x 25cm brownie tin with oil.
Using a hand-held beater, beat together the eggs, oil, buttermilk and sugar in a mixing bowl until well combined.
In a separate bowl sift the flour, cocoa powder and bicarbonate of soda. Fold into the egg mixture then carefully add 250g of the raspberries and fold to combine.
Pour the mixture into the prepared tin and bake on the centre shelf of the oven for approximately 45 minutes or until a cake skewer comes out clean when inserted into the middle of the slice. Remove from the oven and allow to cool for 10 minutes.
Cut into 18 squares then carefully transfer to a wire rack to cool completely.
Serve with the extra berries and a dollop of reduced fat yoghurt.

Chocolate berry slice Image: Louise Lister