Preheat oven to 350°F or 180°C.
Sift the flour and baking powder together in a bowl.
Cream the butter and sugar together.
Beat in the egg, followed by the milk.
Gradually beat in the flour mixture, being sure to scrape the sides of the bowl to get an even mixture.
Place the blackberries in a 1½ quart (1 - 1½ litre) casserole greased with butter.
Mix the cornstarch into the cooled jam syrup till smooth, and pour over the berries.
Use a tablespoon to 'dollop' the batter onto the berries and syrup.
Bake for 40 to 50 minutes until a toothpick inserted into the topping comes out clean.