Set your oven to 190C/170C fan/375F, Gas 5. Put 12 -18 paper cases into muffin tray (depending on the size of your cases).
Sift the dry ingredients into a bowl. Stir in the plums. Beat the honey, milk, egg and oil together and stir into the other ingredients.
Make the topping by mixing the melted butter with the oats and sugar.
Half fill each paper case with cake mixture. Top each cake with one teaspoon of the topping mix.
Bake for 12 to 15 minutes or until risen and set. Test with the point of a skewer inserted in the middle of the cake. If it comes out cleanly the cake is ready.
Cool on a wire rack and enjoy freshly baked.