Preheat the oven to 190°C | Gas mark 5. Line a baking sheet with baking parchment and set aside.
Place 1 filo sheet on your work surface and lightly cover the sheet with oil spray. Sprinkle with 1 tablespoon of sugar, then place another sheet on top. Repeat, layering the remaining filo sheets on top and spraying each with little oil and sprinkling with a tablespoon of sugar in between each one.
Cut the filo pastry lengthwise into 4 equal strips, then cut each strip into 4 equal rectangles. You should end up with 16 rectangles.
Transfer rectangles to the prepared baking sheets and bake in the middle of the oven for 8 minutes or until golden and crisp.
Cool on a wire rack before filling.
Prepare the custard. In a saucepan, mix two tablespoons of custard powder with 1 tsp of vanilla extract and 2 tablespoons of plant milk to form a paste. Add the rest of the milk and bring to a boil over small heat. Once thick, remove from the heat and set aside for 5 minutes to cool before transferring into a piping bag.
To assemble, put a filo rectangle on a serving plate, pipe the custard mixture and add a layer of raspberries. Place another filo rectangle on top, pipe more custard and top with a final filo rectangle. Repeat assembling to make six stacks.
Melt the chocolate and transfer it to a piping bag fitted with a small round nozzle. Set aside.
To make the icing, place the icing sugar into a large bowl. Add 2 tablespoons of water and mix to combine.
Once ready, pour the icing over the top of each mille-feuille and spread evenly using a pallette knife.
Working quickly, pipe three thin chocolate lines along the width of each pastry, then using a toothpick or a sharp knife lightly draw it down (from top to bottom) through the rows of chocolate.
Serve cold or at room temperature.