Transfer the mixture to a bowl. Beat the eggs with the milk and stir in the melted butter. Add the egg mixture to the almond mixture and whisk for a minute or two until the batter is smooth.
Pour the batter into the pan and smooth with a spatula. Arrange the plum slices on top in a circular pattern. Sprinkle sugar generously over the plums.
The cake is best served within a few hours of baking.