Step 1

Preheat oven to 170C/150C fan forced. Spray a square 20cm cake pan with oil and line with baking paper, allowing paper to overhang the sides. Spray the paper with oil and line with another sheet, allowing both sheets to overhang.

Step 2

Use electric beaters to beat the butter and sugar until pale and creamy. Add the egg yolks and vanilla and beat until combined. Sift over the flour and baking powder. Use a flat-bladed knife to mix until the mixture starts to come together. Turn the dough onto a lightly floured work surface and divide into 2 equal portions.

Step 3

Place half the dough in the prepared pan and press to evenly cover the base. Spread the jam over the dough. Roll the remaining pastry out on a sheet of baking paper to a 20cm square. (If the dough is very soft you can chill it until it is easier to work with.) Invert the pastry over the jam and remove the paper. (Don't worry if the dough breaks a little, just gently press it into shape).

Step 4

Bake for 25 minutes or until golden. Cool in the pan for 20 minutes then use the baking paper to remove shortbread from pan and transfer to a wire rack. Remove the paper and set aside to cool completely.

Step 5

For the icing, sift the icing sugar into a bowl. Place the butter on 1 side of the bowl. Pour 1 tablespoon of the boiling water over the butter and stir until butter is melted and the icing is smooth, adding more water, 1 teaspoon at a time, to make a spreadable consistency. Add 1-2 drops of food colouring and stir until combined. Spread the icing over the shortbread then while the icing is still wet, dip a fork into boiling water and use a tea towel to dry it. Run the warm fork through the icing to make lines, heating and drying the fork each time. Set aside for 1 hour or until set. Cut into squares to serve.