Preheat the oven to 190C/170C fan/Gas 5
Rub the butter and flour together until the mixture resembles breadcrumbs, then mix in the remaining crumble ingredients using a fork.
Line a baking tray with greaseproof paper. Spread the crumble mixture onto the lined tray and bake for 20 minutes until golden.
Whilst the crumble is baking. Pop the blackberries and caster sugar in a pan. Cook on a medium heat until sugar dissolves. Turn the heat down and simmer for 5 minutes until the coulis has thickened. Push through a fine sieve into a jug, discard the seeds and remaining pulp. Allow to cool.
When the crumble is golden remove from the oven and allow to cool.
Once the blackberry coulis and crumble have cooled you are ready to create your Blackberry Crumble Sundae. Start by adding a scoop of gelato to each sundae dish, drizzle with some blackberry coulis and top with crumble, repeat and garnish with a couple of fresh blackberries.