Lightly coat or spray a 12-cup muffin tin with oil.
Place 1 phyllo sheet on a large cutting board.
Lightly brush with butter, then sprinkle with 1 1/2 tsp sugar. Cover with another phyllo sheet. Line up all edges.
Repeat brushing, sprinkling and layering with 2 more phyllo sheets, ending with sugar sprinkled overtop.
Cut phyllo stack into 12 equal squares. Carefully place 1 square stack in muffin cup, pressing around bottom with your fingers to form a cup. Leave sides of phyllo upright. Do not fold over.
Repeat with remaining phyllo square stacks. Prick bottoms of cups with a fork.
Bake in centre of preheated oven until crispy and golden, 7 to 9 minutes. Remove to a cooling rack.
In a small saucepan, stir 2 tbsp sugar with custard powder until evenly mixed.
Stir in orange peel.
Gradually stir in about 1/4 cup cream until no lumps remain. Then gradually stir in remaining cream and vanilla.
Place over medium heat. Stir with a wooden spoon until gently boiling and thickened, 5 minutes.
Pour into a medium-size bowl. Press plastic wrap onto surface. Refrigerate until cold, at least 1 hour or overnight.
Place frozen or fresh berries in a large saucepan. If fruit is in a frozen block, it will take longer to cook. You will have to break it up with a spoon.
Add rhubarb. Mix 1/2 cup sugar with cornstarch and stir in until blended.
Place over medium heat. You do not have to add any water – juices come out of the fruit as it cooks.
Stir often until a thick sauce forms, 10 to 15 minutes.
Stir constantly for the last few minutes of cooking. Reduce heat if sauce starts to stick.
To serve - spoon about 2 teaspoons of custard into each phyllo cup. Top with rhubarb mixture. Garnish with a mint or lemon balm leaf.
Note: To prepare this recipe in advance - Prepare phyllo cups, custard and rhubarb sauce. Store cups, covered, at room temperature up to 4 days. Refrigerate sauce, covered, up to 3 days and custard up to 1 day. Fill just before serving.