Cream the butter together with the sugar until soft and creamy.
Add the eggs one by one, beating between each addition.
Add the almond extract and baking powder, then fold in the flour and ground almonds.
Add half of the chopped plums to the mix.
Grease and line your slow cooker pot with baking paper or a large cake case.
Put the mixture in the cake case, then add the remaining plums on the top, pressing them in slightly.
Put on the lid and cook for 1-2 hours on high, until a skewer comes out clean. This took 1.5 hours in my slow cooker. A good tip when making a cake in the slow cooker is to put a tea towel under the lid, to catch water dripping off and stop it making the top of the cake go soggy.
Once done, allow the pot to cool slightly before removing the cake and placing it on a rack to cool.
For the topping, warm the jam in the microwave. Meanwhile, rub the butter, flour and sugar together in a small bowl until it looks like breadcrumbs.
Chop the plums and place them on the cake. Drizzle over the warm jam, then scatter as much of the sugar mixture as desired on top.