Preheat the oven to 170 degrees Celsius (340 F). Line a 20cm x 20cm baking tin.
Combine the dry ingredients from the crumble in a bowl and pour over the melted butter. Use a spoon to ensure they're well combined and none of the flour is left 'dry'.
Spread three quarters of the mixture in the bottom of the baking tin, applying pressure to pack it together. Bake for 10 mins or until the top starts to turn golden brown.
Mix the plum layer ingredients together, adding an even layer to the base. Top with the remaining crumble mixture. Bake for around 20 mins or until the top has browned again.
Allow to cool before cutting or attempting to take it out of the tin.
Recipe by A Life of Geekery at http://www.alifeofgeekery.co.uk/2013/09/plum-crumble-bars.html