For puree, make a paste of cornflour and vanilla, and add to blender with berries and jam. Puree until smooth. Set aside.
Preheat oven to 170°C. Grease a 23cm-springform pan.
Combine caster sugar, flour, baking powder and bicarbonate of soda in a large bowl. Rub in butter cubes with your fingertips until the mixture resembles coarse breadcrumbs. Remove ½ cup and set aside, then add sour cream, egg and vanilla to main bowl. Mix well with a wooden spoon.
Using a little over half the cake batter drop dollops of batten into pan. Pat batter across bottom of pan and about 2.5cm up sides; mixture will be sticky and uneven. Add strawberry puree, making an even layer across bottom of pan and leaving a rim of dough above it cover with remaining cake mixture.
Use a fork to combine reserved flour and butter mixture with demerara sugar. Sprinkle evenly over cake, then bake for 45 minutes or until lightly golden. Cool in pan, then serve.