Preheat the oven to 180 C / Gas 4. Grease and line a 20cm cake tin with baking parchment.
In a large mixing bowl, cream the margarine and sugar together until pale and fluffy. Beat in the eggs, one at a time, until the mixture is creamy and well combined. Add the vanilla extract and mix well.
In a separate bowl, sift in the flour then add the ground almonds and baking powder. Fold the dry ingredients into the wet ingredients and mix well. Add the rhubarb and mix well. Transfer the mixture to the prepared cake tin and smooth the top. Sprinkle the flaked almonds on top.
Bake in the preheated oven for 1 hour or until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool in the tin.
When ready to serve, remove from the tin using the baking parchment to help you. Place on a serving plate and spread the rhubarb jam thinly on top. Slice and serve.