Preheat the oven to 350 °F / 180 °C / Gas Mark 4, no fan.
Prepare all the ingredients.
Prepare a 9x5-inch loaf pan (23x11cm)* by buttering it and lining with parchment paper.
In a small pot, melt the butter, take it off the heat, and leave to cool.
In a small bowl, mix the flour, salt, baking powder, and baking soda together. Set aside.
Add the eggs and sugar to a mixing bowl, whisk with a mixer fitted with a whisk attachment on medium-high speed for 5 minutes until the mixture is pale, light, and foamy.
Add the mashed bananas, vanilla extract, and melted and slightly cooled butter, and mix until just combined.
Sieve the flour mixture into the batter, stir in with a spatula, until partially combined.
Add the chocolate (broken into smaller pieces) and a little over a cup of blackberries, fold in, taking care not to overmix.
Pour batter into the prepared loaf pan.
Lay the remaining blackberries on top.
Bake for about 1 hour, or until a cake tester comes out clean. It may run longer or shorter in your oven. After about 50 minutes, check if the top is not browning too much, if it is - cover it loosely with a piece of aluminum foil.
Enjoy!