Place a medium saucepan over medium heat and add the berries, 0 cup cinnamon, 2 tablespoons sugar, and water and stir frequently.
Once the mixture starts boiling, reduce the heat and let it simmer for around 8 minutes or until the mixture starts to thicken and then turn off the heat.
Meanwhile, combine the oats, flour, remaining cinnamon, and baking soda in a mixing bowl.
Slowly stir in the melted butter and mix well until the ingredients are very well combined.
Divide the flour mixture into two and set one aside, this will form the topping.
Scoop the remaining mixture into an ungreased baking pan (9” square pan). Bake for around 25 minutes then remove from oven.
Evenly spread the berry mixture on the baked crust and top with the reserved flour mixture. Press the flour mixture lightly using your fingers and return to the oven and bake for a further 20-25 minutes until the topping starts browning.
Remove the pan from the oven and allow to cool on a wire rack, then cut the blackberry muesli bar into desired sizes and shapes.