Wash the fresh plums under cool water, then halve and pit them. Set aside while you preheat the oven.

Preheat your oven to 360°F (180°C) and grease a 10x8" baking pan. In a bowl, combine all dry ingredients: flour, sugar, baking powder, and baking soda. Whisk to blend, then add coconut oil, coconut vinegar, vanilla extract, and lukewarm water, mixing until smooth.

In a separate bowl, combine cold vegan butter, flour, sugar, and cinnamon powder using a pastry cutter or your fingers until crumbly.

Pour the cake batter into the greased baking pan, spread evenly, place halved plums on top, and sprinkle the streusel over plums. Bake for approximately 25 minutes.

Let the cake cool in the pan for about 10 minutes before transferring it to a wire rack. Serve slightly warm.