Preheat the oven to 180°C.
Wash the plums, remove the stones and cut them into smaller pieces.
In a bowl, combine all the crumble ingredients: chopped butter, instant oats, sugar and chopped pecans. Set the prepared crumble aside.
In a medium bowl mix dry ingredients (except for the sugar): wheat flour, baking powder and baking soda.
Using a stand mixer (or hand mixer) beat the whole eggs until fluffy (about 6-8 minutes). Continue mixing and add the sugar, spoon at the time (about 3-4 minutes). Continue mixing and very slowly pour the oil into the batter.
Reduce the speed to low, add the dry ingredients and mix just until combined. Finally, slowly pour in the milk and mix until a smooth batter is formed.
Pour the batter into a round baking tin with a removable bottom, lined with baking paper. Put the plum pieces on top. Finally, cover everything with the crumble. Bake your cake for about 50-60 minutes or until a dry toothpick. It's best to check before the end of baking and adjust the time if necessary.