1. Preheat oven to 170Â°C. Cream 250g of the butter and both sugars until fluffy. Stir in vanilla and add eggs one at a time, mixing well between each addition.
2. Fold the flour into the creamed mixture until it starts to pull together. Mix through the choc chips. Carefully spread cake batter between 2 (25cm x 15cm) slice pans, smooth out and bake for 20 minutes.
3. Cool for 5 minutes in the pan. Cut out 9 triangles per slice pan, using the measurements in the biscuit template as a guide. Continue to cool the triangles in the pan.
4. Combine sugar and 30g of butter, mixing to a smooth thick icing with boiling water. Gently lift out each triangle, dust the underside with icing sugar then build 2 “trees” by placing the largest triangle underside up, at the base, pipe some butter icing in the centre and gently press another triangle on top. Continue stacking up to the smaller triangles at the top, using the butter icing between each biscuit to hold them firm.
5. Decorate with Smarties secured by the butter icing. Pull apart to serve.