In a mixing bowl, whisk together custard powder and sugar until combined. Gradually add half of the dairy-free milk, stirring until smooth. Pour into a saucepan with remaining milk, heating over medium-high while stirring continuously. Cook until thickened, about 5-7 minutes. Remove from heat, stir in vegan butter and vanilla, then chill for 2-4 hours.

Preheat oven to 200°C (390°F) and line baking trays with parchment paper. Roll out the frozen puff pastry on a floured surface. Brush one sheet with maple syrup, layer another on top, and cut into squares, scoring smaller squares for decoration. Brush tops with more maple syrup.

Place the pastry squares on the lined tray and bake for 15-20 minutes, until puffed and golden brown. Let cool completely on a wire rack.

Once cool, cut along scored lines to create pockets. Add a spoonful of jam, followed by chilled custard filling. Top with fresh berries and dust with powdered sugar if desired.

Serve within 1-2 hours of assembling for the best texture. Sprinkle dried rose petals for an exquisite touch.