Preheat the oven to 350 degrees Fahrenheit.
Wash, dry, and thinly slice the plums.
Add the unsweetened almond milk, maple syrup, almond butter, ground flaxseed, pure vanilla extract, and salt to a large mixing bowl and use a whisk to mix well.
Add the coconut flour and baking powder, then use a spoon to stir until combined.
Grease the sides of a 9 inch baking pan with a small amount of oil, then place a piece of parchment paper large enough to cover the entire bottom of the pan.
Transfer the cake batter to the baking pan and use a spatula or the back of a spoon to spread the batter into an even layer.
Place the sliced plums on top of the batter (you can create a pretty pattern with the plum slices or just place them on top randomly for a more rustic look).
Bake at 350 degrees Fahrenheit for 45 minutes.
Let the cake cool before serving.