Place milk and cream in a medium saucepan over medium heat. Cook, stirring continuously, for 10 minutes OR until almost simmering (don't boil).
In a separate bowl, beat yolks, sugar and vanilla with an electric mixer until pale and thick. Add corn flour and custard flour and mix until smooth. Gradually beat in milk mixture. Empty mixture back into the saucepan and cook, stirring continuously, over a low heat for 6 minutes or until mixture thickens and coats the back of a spoon. Dissolve the gelatine powder in a little hot water and add to thickened mixture. Mix thoroughly. Remove from heat and add the butter, stirring until melted and smooth. Place custard in a bowl and cool slightly. Cover surface with cling film and set aside for 30 minutes to cool.
Preheat oven to 180oC fan-forced. Line two baking trays with baking paper. Place one pastry sheet onto each tray. Bake for 15 to 20 minutes or until golden brown. Cool for 5 minutes. Cover each sheet with a clean tea towel and using your hands, gently press pastry to flatten.
Lightly grease a 6cm deep, 23cm (base) square pan. Line base and sides with baking paper, allowing a 3cm overhang on all sides. Trim and place one pastry sheet on the bottom of the prepared pan. Pour custard over pastry. Top with remaining trimmed pastry sheet. Cover and refrigerate for 4 hours or until set.
Prior to cutting, dust with icing sugar.
Use a serrated knife to cut into squares, being careful not to place to much pressure on the slice.