Cake: Preheat the oven to 170 degrees C. Grease and line two 20cm round cake tins.
Sift the flour and baking powder into a large mixing bowl. In a separate bowl, cream together the butter, sugar and pumpkin extract until fluffy.
Add the eggs, one at a time, until pale and creamy. Fold the flour and baking powder into the butter mixture until well combined. Divide the mixture equally between the two prepared cake tins.
Bake in the preheated oven for 30 to 35 minutes or until skewer inserted into the centre comes out clean. Remove from the oven and allow to cool completely on cooling racks.
Buttercream Icing: In a mixing bowl, beat the butter and half the icing sugar until smooth. Add the rest of the icing sugar and 1 to 2 tablespoons milk until the consistency is thick but spreadable.
To Assemble: Place the first sponge on a serving plate and spread the plum jam over the top. Top with a layer of the buttercream icing then sandwich the second cake layer on top. Dust with icing sugar to decorate.
Use vanilla extract if you cannot find pumpkin extract.