Prepare the dry ingredient mix by whisking together, in a medium bowl, the rest of the sugar, the flour, baking powder, salt, and cardamom. Set this aside.

Prepare the wet ingredients in a medium bowl by gently whisking together the eggs, vanilla, milk, and melted butter.

Pour the wet ingredients over the dry ingredients, and stir to combine.

Divide the batter evenly by filling each of the muffin cups at least halfway with batter. Then distribute the plum chunks evenly over the batter in the muffin cups, and top with another dollop of batter. Sprinkle the tops of each muffin with a little of the cinnamon sugar mixture (you will probably have lots of cinnamon sugar left. Sprinkle it on buttered toast for breakfast!).

Fill the remaining empty muffin cups halfway with water, transfer the pans to the oven, and bake for about 18 minutes. Check to see if they’re done with a cake tester. It should come out clean when the muffins are done.

Transfer the pans to a rack and cool for about 10 minutes before unmolding the muffins.