Preheat the oven to 180°C. Grease a 25 x 15cm baking tin and line the base with paper.
Cream the butter and sugar together until combined. Mix in the marzipan.
Add the egg and almond essence and mix to combine. Sift in the flour, baking powder and salt and mix until everything comes together.
Gently press the dough into the baking tin. Use a fork to mark lines on the surface of the dough down the length of the pan. Bake for 25 minutes or until golden. Allow to cool completely before removing from the pan and cutting into squares.