Preheat the oven to 350 F.
Coat the bottom of a 8" cake or springform pan with ½ tablespoons of the melted oil. Sprinkle the sugar evenly over the oil to cover the base of the pan. Lay the thin plum slices on the bottom on the pan in overlapping concentric rings.
Place flour, baking powder, baking soda, spices, pepper, and salt in a large bowl. Whisk to combine.
Melt the remaining 6 tablespoons of coconut oil. Add in the maple syrup, milk, and vanilla. Whisk to combine.
Pour the wet ingredients into the dry ingredients bowl, whisk together to remove lumps. Add the vinegar, and whisk vigorously to incorporate.
Pour the battered over the prepared cake pan with the plums and caramel sauce. If using a springform pan, place it on a rimmed baking sheet so as to catch any leaks.
Bake in the center of the oven, until a toothpick comes out clean, about 40-47 minutes for an 8" pan.
Cool the cake on a wire rack for 10-15 minutes. Invert a rimmed plate or serving dish on top of the cake pan, then swiftly invert both such that the caramelized plums are on top of the cake. Serve immediately (while still warm) or serve at room temperature.
Store in the fridge for up to two days.