Line a square cake tin with cling film and lightly spray it with oil.
Prepare another layer of cling film on the bench and light spray with oil.
Place the shortcrust pastry in a blender and blitz it into fine crumbs.
In a medium saucepan over a medium heat, bring the milk and Shape It Powder to the boil and simmer the mixture gently for 1 minute, stirring occasionally.
Pour the hot milk-Shape It mixture over the crumbs and blend at a high speed for 2–3 minutes, or until smooth.
Spread half this mixture into the bottom of the prepared cake tin to a thickness of 5mm (for the base).
Spread the other half of the mixture to the same thickness over the other prepared cling film (for the lattice).
Cover both with cling film and refrigerate for 2 hours, or until they set.
To cut the lattice, cut the cooled shortcrust into 2cm widepieces and the same length as the base.
Make a lattice shape with the shortcrust strips; this will sit on top of the finished pie.