Place the blackberries in a bowl with 2 tablespoon of the caster sugar and the cassis or brandy. Stir to combine and set aside while preparing the batter.
Preheat the oven to 180℃ (170℃ fan)/350°F/gas mark 4. Generously butter a shallow oven proof dish then add 1 tablespoon of caster sugar and tilt the dish until the bottom and sides are coated with caster sugar. Set aside.
Sift the flour into a bowl and make a dip in the centre. Drop in the eggs and a splash of milk, whisk the eggs and milk together gradually blending in the flour to create a smooth batter, then whisk in the remaining milk.
Melt the butter and whisk into the batter.
Place the blackberries into the prepared ovenproof dish, then pour in the batter. Bake for 30 minutes or until the just set.
Serve warm or at room temperature, dusted with icing sugar if desired.