Place the peach halves skin side down onto a baking tray lined with parchment, sprinkle with some of the caster sugar and place into the preheated oven for 10 to 15 minutes. When soft and tender, take out of oven and leave to cool down before removing the skins and slicing into threes.
Grease and line the base of a 20x30cm (8x12in) baking tray.
Sift the flour and baking powder into a large bowl and then rub in the butter with your fingertips. Stir in both the sugars.
Press half the mixture over the base of the tin.
Add the vanilla seeds, extra baking powder, flour, egg and milk to the rest of the base mixture and stir well until a batter consistency is formed.
Pour the batter evenly over the base, lay the peaches over the top and sprinkle with the raspberries. Place into the preheated oven and bake for 35 minutes.
Cool the peach and raspberry tray bake in the tray and then cut into squares to serve.