Butter eight 8 oz. ramekins. Heat the oven to 350°F. In a medium saucepan over medium heat, combine 6 oz. (1-1/2 sticks) of the butter with the brown sugar, whisking until smooth and combined (you’ll still feel sugar granules). Immediately pour into the bottoms of the buttered ramekins, dividing evenly. Arrange a layer of plum slices in each ramekin, overlapping them slightly, using smaller slices to fill any gaps.
In a small bowl, combine the milk and lemon juice (the mixture will curdle; this is okay). In another bowl, sift together the cake flour, baking powder, baking soda, cinnamon, and salt.
With an electric mixer, cream the remaining 4 oz. (1 stick) butter with the sugar and the orange zest. Add the eggs and vanilla; mix to combine. Alternately add the dry ingredients and the milk mixture, mixing just to combine. Divide the batter among the ramekins.Arrange the ramekins on a rimmed baking sheet and bake until the cakes are firm to the touch and the juices are bubbling, 35 to 40 minutes, rotating the sheet midway through baking to ensure even cooking. (If you insert a skewer to test doneness, it may be moist with plum juices but should show no raw batter). Let cool on a rack until warm. Run a paring knife around the inside edge of each ramekin, invert onto a plate, and gently lift it off the cake. Top each cake with a scoop of vanilla ice cream, if you like. If you’ve baked the cakes ahead, heat the oven to 350°F. Set the ramekins on a rimmed baking sheet and heat until just warmed, about 2 minutes.