Combine butter, flour, sugar and salt in a bowl, and use your fingers or a pastry cutter to work mixture until butter is the size of small peas and dry ingredients have been absorbed. Sprinkle into prepared pan and press down with your fingers to make an even layer. Bake for 18 to 20 min or until crust is lightly golden. Turn oven down to 350F.
Combine sugar, flour, cinnamon and butter in a bowl and work with your fingers until it has come together in a crumbly mixture. Add almonds. Cut prune plums in half lengthwise and remove pits. Place plums on the crust in neat rows, alternating cut side up and down, and sprinkle almond mixture overtop.
Bake for 55 min or until plum juices are bubbling and syrupy. Cool and cut into squares.