Preheat your oven to 170°C. Grease and line your baking tin.
Start by making the base. Cream the butter and castor sugar together until pale and fluffy. Add the flour, cornflour and baking powder. Beat for a minute, then use your hands to bring the mixture together.
Press the dough into the tin in an even layer. Spread the apricot jam over the shortbread base layer. Set the tray aside.
Place all of the topping ingredients in a bowl. Use your fingertips to rub everything together until evenly combined. Crumble small clumps of the topping over the jam layer as evenly as you can.
Pop the tin in the oven and bake for 40 minutes until the topping is golden. Allow to cool completely before removing from the tin and cutting into squares.