Preheat oven to 350ºF. Grease a 9-inch removable bottom cake pan or springform pan with butter.

To make the cake, whisk the flour, coconut, baking powder, and salt in a bowl. Set this dry mix aside.

In the bowl of a standing mixer fitted with the paddle attachment, cream the butter and granulated sugar on medium-high speed for 5 minutes. Scrape down the sides of the bowl and beat for 2 more minutes.

Decrease the speed to medium-low and add the eggs one at a time, beating well after each addition. Add the almond extract and turn the speed down to low.

With the mixer still running, add half the dry ingredients to the wet, followed by all of the milk and then the rest of the dry ingredients. Mix until combined, scraping the bowl once or twice.

Transfer the batter to the prepared pan. Use a spatula to spread the thick batter to the edges of the pan. Stand the plums in rings in the batter, up on their ends.

Combine the demerara sugar and cinnamon in a bowl, then sprinkle across the fruited Stonehenge. Bake for 50 minutes.

While the cake bakes, make the topping. Whisk the butter, sugar, eggs, almond meal, vanilla bean seeds, almond extract, and shredded coconut in a pitcher with a pouring spout.

Working quickly, remove the cake from the oven and pour the mixture over the hot cake. Return the cake to the oven and bake until the topping is puffed, evenly golden, and set, 25 to 30 minutes more.

Cool the cake completely, in its pan on a wire rack before serving. Serve as is or dusted with confectioners' sugar.