Preheat oven to 350°F.
Line springform pan with parchment. Pour 2 T melted butter onto the parchment and brush a little butter onto the sides of the pan. Add honey to the butter on the bottom of the pan and stir to mix. It doesn't have to be perfectly mixed.
Lay the plum slices around the pan on top of the honey and butter mixture. You could do this randomly, or lay them in a circle in a pretty pattern. Try to have them cover the bottom of the pan evenly.
In a bowl, combine the eggs, remaining honey and melted butter, vanilla, baking soda, and cinnamon. Mix thoroughly, then add coconut flour and mix until combined.
Carefully pour the batter over the plums, trying not to disturb their arrangement.
Bake 35-45 minutes at 350°F, until a toothpick inserted into the cake emerges dry. A crumb or two is fine.
Let the cake rest for 10 minutes or so. Then cover with a serving plate, turn over, and remove the sides of the springform pan. Remove the bottom of the pan and carefully peel the paper off.
Alternatively, you can remove the sides, then invert, then take the bottom and paper off. It also works fine to use a non-springform pan and invert the same way.
Serve plain, or add cultured cream, whipped cream, whipped coconut cream, or ice cream.