For the filling, in a large lidded saute or frying pan, heat the butter until fairly hot and starting to brown. Toss in the apple and fry for 30 seconds or until starting to brown. Toss 100g of the sugar together with the apple over a medium-high heat, then reduce to low and add 2 tbsp water. Cover with a lid and gently steam for about 3-4 minutes until just soft. Take off the heat and stir in the blackberries, and leave covered for a few more minutes. Check for sweetness, adding a little extra sugar if needed. Uncover to cool completely.