Preheat your oven to 180 °C (350 °F). Line a 9 inch diameter baking dish (26 cm) with parchment paper or lightly grease it.
Remove the seeds and quarter the plums.
In a large mixing bowl, combine: flour, ground walnuts, sugar, baking powder, baking soda, salt, cinnamon (if using). Whisk everything together until evenly mixed.
In a separate bowl, whisk together: plant milk, oil, apple cider vinegar or lemon juice, vanilla extract. Pour the wet mixture into the dry ingredients and stir gently until just combined. Don’t overmix, the batter should stay light.
Spread the batter evenly into your prepared baking dish using a spatula. Place the plum quarters on top of the batter, skin side down in tight rows to cover the surface.
In a bowl, combine: flour, sugar, walnuts and butter. Rub the mixture together with your fingers until crumbly and generously sprinkle it over the plums before baking.
Bake for 40–50 minutes, or until the top is golden brown. Let it cool off and enjoy!