Preheat the oven to 180C/350F. Line a 9 inX13 (or 9inX9in) in pan with parchment paper and set aside.
Put all the ingredients for the pastry in a food processor and pulse until well blended. Alternatively you can mix the pastry with fork. Tip the rough pastry/crumbs into the lined pan and press firmly with the back of a spoon to form the base. Prick with a fork and bake in the preheated oven for 15 minutes.
Meanwhile prepare the crumb topping by pulsing the ingredients in a food processor until butter is evenly distributed and the mixture resembles coarse crumbs.
Grate the marzipan, it's easier to do if it's slightly chilled.
Take the pan with the base out of the oven, spread the blackberries (no need to defrost them if using frozen), then sprinkle with grated marzipan and the crumb topping.
Bake for 20 minutes until golden. Let it cool in the pan, then cut into bars.