Preheat oven to 160 degrees. I double the ingredients when I use an oven tray. Cream butter & sugar. Add oil & vanilla & mix well. Stir in beaten egg. Add baking powder & flour. Mix all together into a dough & knead well. Spread dough into a greased tray & lightly prick with a fork. Spread softened jam over dough. Repeat mixture as above & grate onto jam. Bake for approximately 30 to 40 minutes. Remove when tart has risen & light brown. Dust top with icing sugar whilst hot. Cut into squares when cool.