Butter, essentially. You need the perfect ratio of butter to dry ingredients; it aids the crumble to brown and get crisp. We recommend using slightly softened butter and rubbing in the dry ingredients thoroughly as this makes it extra clumpy. Leave your butter out of the fridge for 20-30 minutes at room temperature to allow for this or for slightly less time if in a hot climate. Cold butter will make far finer crumbs. Ensure the butter is combined into the dry ingredients properly otherwise you will end up with sandy dust, not crumble. It needs to be the perfect balance of wet and dry so ensure you follow the ratios in the recipe card accurately. It should be like slightly sticky clumps of crumble. If you use cold butter and your crumble feels a little dry, you can add a splash of milk or cream to bring it together.