Heat the oven to 180C/160C fan/ gas 4. Butter a 20cm square cake tin and line with baking parchment. Combine the flour, baking powder and oats in a large bowl. Set aside.

Tip the butter, sugar and molasses into a saucepan over a medium-low heat and stir for a few minutes until just melted. Remove from the heat and pour into the dry ingredients, then stir to combine.

Spoon half the crumble mixture into the prepared tin, pressing it down in an even layer and making sure the surface is level.

Scatter over the hazelnuts and blackberries, then spoon over the remaining crumble mixture – no need to level it out too much. Bake for 30 mins until golden, then leave to cool in the tin for 10 mins. Cut into squares to serve. Will keep in an airtight container for up to five days.